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Linux Swap filesystem

News Linux Disk Management Recommended Links Swap file /etc/fstab/
Linux Disk Partitioning   Filesystems Recovery Humor Etc
Generally with huge memory space on modern computers role of swap is diminished. In most cases on servers with memory above 8G you can create swap partition equal to the memory. There are, of course, exceptions. Oracle is one such exception and Oracle installer generally requires 2x physical RAM amount of swap. You can provide this amount of space in several swap partitions (one of each mirrored pair of disks) as allocating large swap partition of a single disk is pretty taxing (database servers usually have at least two or three pair of mirrored disks).
General rule from Red Hat states that 

Swap should equal 2x physical RAM for up to 2 GB of physical RAM, and then an additional 1x physical RAM for any amount above 2 GB

For systems with really large amounts of RAM (more than 32 GB) you can likely get away with a smaller swap partition (around 1x, or less, of physical RAM).

As for whether to use swapfile or swap partition on desktops with low loads it does not matter and swap file in more convenient (typically Linux desktops has one large or two large partition which increase efficiency of space utilization and simplify backup -- tradition that came from Windows) .  See discussion file vs swap partition for some useful additional considerations.

Here is full text of Red Hat  recommendations (Partitionining of x86, AMD64, and Intel® 64 systems)

Unless you have a reason for doing otherwise, we recommend that you create the following partitions for x86, AMD64, and Intel® 64 systems:

Another useful discussion can be found at All about Linux swap space  by Gary Sims

Linux has two forms of swap space: the swap partition and the swap file. The swap partition is an independent section of the hard disk used solely for swapping; no other files can reside there. The swap file is a special file in the filesystem that resides amongst your system and data files.

To see what swap space you have, use the command swapon -s. The output will look something like this:

Filename        Type            Size    Used    Priority
/dev/sda5       partition       859436  0       -1

Each line lists a separate swap space being used by the system. Here, the 'Type' field indicates that this swap space is a partition rather than a file, and from 'Filename' we see that it is on the disk sda5. The 'Size' is listed in kilobytes, and the 'Used' field tells us how many kilobytes of swap space has been used (in this case none). 'Priority' tells Linux which swap space to use first.

One great thing about the Linux swapping subsystem is that if you mount two (or more) swap spaces (preferably on two different devices) with the same priority, Linux will interleave its swapping activity between them, which can greatly increase swapping performance.

To add an extra swap partition to your system, you first need to prepare it. Step one is to ensure that the partition is marked as a swap partition and step two is to make the swap filesystem. To check that the partition is marked for swap, run as root:

fdisk -l /dev/hdb

Replace /dev/hdb with the device of the hard disk on your system with the swap partition on it. You should see output that looks like this:

 Device Boot    Start   End     Blocks  Id      System
/dev/hdb1       2328    2434    859446  82      Linux swap / Solaris

If the partition isn't marked as swap you will need to alter it by running fdisk and using the 't' menu option. Be careful when working with partitions -- you don't want to delete important partitions by mistake or change the id of your system partition to swap by mistake. All data on a swap partition will be lost, so double-check every change you make. Also note that Solaris uses the same ID as Linux swap space for its partitions, so be careful not to kill your Solaris partitions by mistake.

Once a partition is marked as swap, you need to prepare it using the mkswap (make swap) command as root:

mkswap /dev/hdb1

If you see no errors, your swap space is ready to use. To activate it immediately, type:

swapon /dev/hdb1

You can verify that it is being used by running swapon -s. To mount the swap space automatically at boot time, you must add an entry to the /etc/fstab file, which contains a list of filesystems and swap spaces that need to be mounted at boot up. The format of each line is:

<file system>     <mount point>     <type>     <options>        <dump>    <pass>

Since swap space is a special type of filesystem, many of these parameters aren't applicable. For swap space, add:

/dev/hdb1       none    swap    sw      0       0

where /dev/hdb1 is the swap partition. It doesn't have a specific mount point, hence none. It is of type swap with options of sw, and the last two parameters aren't used so they are entered as 0.

To check that your swap space is being automatically mounted without having to reboot, you can run the swapoff -a command (which turns off all swap spaces

My additional information:

<root /># free -m
total used free shared buffers cached
Mem: 16023 3128 12895 0 256 2229
-/+ buffers/cache: 641 15381
Swap: 32773 0 32773

How big should my swap space be?

It is possible to run a Linux system without a swap space, and the system will run well if you have a large amount of memory -- but if you run out of physical memory then the system will crash, as it has nothing else it can do, so it is advisable to have a swap space, especially since disk space is relatively cheap.

A rule of thumb is as follows:

The Linux 2.6 kernel added a new kernel parameter called swappiness to let administrators tweak the way Linux swaps. It is a number from 0 to 100. In essence, higher values lead to more pages being swapped, and lower values lead to more applications being kept in memory, even if they are idle. Kernel maintainer Andrew Morton has said that he runs his desktop machines with a swappiness of 100, stating that "My point is that decreasing the tendency of the kernel to swap stuff out is wrong. You really don't want hundreds of megabytes of BloatyApp's untouched memory floating about in the machine. Get it out on the disk, use the memory for something useful."

One downside to Morton's idea is that if memory is swapped out too quickly then application response time drops, because when the application's window is clicked the system has to swap the application back into memory, which will make it feel slow. The default value for swappiness is 60. You can alter it temporarily (until you next reboot) by typing as root:

echo 50 > /proc/sys/vm/swappiness

If you want to alter it permanently then you need to change the vm.swappiness parameter in the /etc/sysctl.conf file.

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[Jan 26, 2011] file vs swap partition

September 17, 2009 | Answer swap

celsius thomas

I have read here (Linux Swap filesystem)

that swapping to a file is slower than swapping to a partition , based on the following reasons

1) large files(swap file) will be somewhat fragmented forcing additional disk/head movement in some cases and that you will have to deal with metadata describing where on the disk the file blocks are. this eats up both in-system filesystem cache and causes additional disk activity while you load metadata that is not in cache

is there any other reason that justifies this claim OR what exactly is the tradeoff between a swap partition and a swap file ??

Sidney Lambe:

Speed is the only issue I've read about in this area. Swap files are often said to be slower than swap partitions.

(Always assumed it had something to do with the difference between a file and a device.)

However, I just reviewed the swapon man page and don't see any mention of this issue.

I'd go with a partition if you have the option.

Keith Keller:

Well, in general (which the above touches on), with a swap file the kernel has to deal with the overhead of the filesystem, whereas with a partition there's no intervening filesystem to get in the way.

But either way, if you're doing a lot of swapping, to the point of thrashing, the efficiency of your swap space isn't going to be a primary concern. :) I think you should prefer a swap partition for organizational reasons, but if you simply can't for some reason a swap file is fine.

September Storm:

My preference is based on error modes not on performance issues -

Using a swap file if it is not available until the file system is mounted. If the file system is corrupted or lost the swap file is not available. A swap file can get included in backups if you are not careful about exclude lists. Also a swap file is unavailable if the drive has a hardware problem or corrupt label. If the file is deleted the inode stays active so the space stays used but it goes away at the next reboot by which time you may have forgotten about an accidental deletion. The swap file can also be renamed making it unavailable at the next reboot.

Using a swap partition none of those issues apply. It's only unavailable if the drive has hardware problems or if the label is corrupted. The list of problems that make it unavailable is much shorter.

MELBIN MATHEW:

I would like to share some difference between swap file and swap partition.

Swap File, 1. If the hard disk is full or corrupted,swap file will get damaged. 2. System speed get decrease as soon the hard disk space is full. 3. It should be on a particular location where it should not get damaged or accidentally copied with other files. 4. It have advantage to increase the swap space on a system that have already installed with linux. 5. In any case if we need to increase the swap space of the system immediately we can do it. 6. We can able to create and keep the swap file on external device eg: Express Card SSD 7. New kernel have nearly achieved to make both swap partitions and swap file speed closer. 8. Swap file will get fragmented.

Swap Partition 1. Reside on a separate hard disk space. 2. Multiple Os on a single machine can share the same partition. 3. Fragmentation is less compare to swap file. 4. If hard disk is corrupted the swap partition would not be functioning. 5. Reduce accident loss or corruption.

*Choice depend on the type of activity we are doing on the system. Increasing Ram will increase the speed of the machine than the swap system. But product price will be high.

Cheers! Melbin Mathew www.talk2melbin.com

[Aug 12, 2010]   Learn Linux, 101 Create partitions and filesystems

Creating swap space

Now let's create some swap space on the /dev/sda4 partition using the mkswap command as shown in Listing 16.

Listing 16. Creating swap space
[root@echidna ~]# mkswap /dev/sda4
Setting up swapspace version 1, size = 4192960 KiB
no label, UUID=8f5a3a05-73ef-4c78-bc56-0e9b1bcc7fdb

Note that recent versions of mkswap show you the generated UUID.

Unlike regular filesystems, swap partitions aren't mounted. Instead, they are enabled using the swapon command. Your Linux system's startup scripts will take care of automatically enabling your swap partitions.

All About Linux Swap Part 2 Management Idea Excursion

Note: This article is the second in a multipart series introducing the Linux swap. Part 1 is intended to familiarize the reader with the concept of swapping, why it exists, and what it’s used for. Part 2 appears below and highlights basic analysis and management techniques for handling swap space. Part 3 will discuss the current state of swap usage and present opinions about its implementation today.

Exploring Swap

If you’re new to Linux and choose a guided installation where the partitioning was automatically configured for you, you may not even be aware of how much swap space you’ve allocated and where exactly it lives. Fortunately, there are a couple different ways that allows us to explore, analyze and manage this information.

swapon/swapoff

swapon -s

swapon is part of a pair of commands (its complement being swapoff) intended to wholly toggle the state of swap. Specifically, man states, “enable/disable devices and files for paging and swapping”. The -s parameter simply displays current usage. The output should look similar to this:

Filename                                Type            Size    Used    Priority
/dev/sda3                               partition       2000084 5452    -1

This result tells us a few things:

More information on priority: A scenario that might exist in which you would need this, is if you decide to add another swap, but have it within a file rather than partition new space for it. You can set the partition to a priority of 5 and the new swap file to Priority of 1. Because the highest value is always preferred, this will ensure that the swap partition is used before the swap file. Another case is spreading the swap over multiple separate drives. If the priority is set the same for both swap devices, the kernel will utilize them in a manner similar to a striped array (round-robin). When it comes time to write to disk, this can help increase performance a bit.

free

Another utility, free, doesn’t give us anymore information than swapon -s, but it allows us to see swap usage in the context of overall system memory.

free -mot

The -m parameter simply displays the values in MB rather than KB. -t gives us a grand total at the bottom. The -o option hides some information we’ll get to shortly. The output might look something like this:

             total       used       free     shared    buffers     cached
Mem:          3959       3934         24          0        158       1957
Swap:         1953          5       1947
Total:        5912       3940       1972

There are 3 lines in the output: Memory, Swap, and the aggregate Total. This gives a better overall picture as to what memory is allocated where. Most of the lines are self-explanatory, but it’s important to note that the very low value of free memory is somewhat misleading. In this example, there is usable free memory of 2140MB.

Without getting too far into the details, think of this as saving a seat next to you in the movie theater. Maybe a friend was joining you for a movie, but in an emergency they had to leave during the previews. Most people assume this seat is reserved, until the theater begins to reach capacity, in which case the seat might really be needed. This extra seat is analogous to cached memory. It was in use at one time, and is still being saved in case your friend comes back, but could be used by another person. Modern OS’s handle memory the same way: They mark the space as used, but available.

To get a simpler picture of how much space we have, we need to add the buffers and cached columns back into the free column, because after all, the seat is technically empty. We can accomplish this automatically by removing the -o and -t parameter from our example (free -m):

             total       used       free     shared    buffers     cached
Mem:          3959       3934         24          0        158       1957
-/+ buffers/cache:       1818       2140
Swap:         1953          5       1947

Now, compare these values to the swap space. Although there is plenty of memory free, some swap is being utilized - a piddly 5MB of 1953MB - nothing to worry about. The final line, total sums things up, indicating we have 5912MB of usable space between all memory and swap, of which 3940 is in use, leaving 1972MB.

The -/+ buffers/cache sub-item simply summarizes how much used or free memory we actually have, and it becomes clear that there is plenty of available room for new applications. So why is swap being used up at all? Without exploring the gritty details, I can offer a much simpler solution: adjust how quickly swap is utilized.

Controlling swap

Because swap is intended as a supplement to memory, the process by which items are swapped in and out is automatic, giving the user little choice what goes into it. However, since kernel 2.6 a small tweak was added to configure how quickly the system swaps items - swappiness.

Swappiness is an arbitrary number from 0 to 100 that indicates how fast you want the system to page items out to disk, freeing up precious RAM. The higher the number, the sooner pages will be written out to swap. You can alter swappiness either temporarily or permanently, as outlined below. Before you go about that, check the value currently set:

sysctl vm.swappiness

Temporary

A simple terminal command is all that is required to change swappiness for the current session. If you want to keep things in memory longer than the current value provides, try lowering swappiness. For example, setting a value of 40:

sudo sysctl vm.swappiness=40

The nice thing about this, is that you can change the setting without rebooting and find a nice threshold that suits your needs. After restarting, the settings will be returned to the default value, so once you find a preferred value, you may decide to change swappiness permanently.

Permanent

This is only slightly harder, but not by much. It actually involved adding or editing a parameter in a single file.

sudo nano /etc/sysctl.conf

Then press Ctrl+W to search for “vm.swappiness”. If nano finds it, great! Just change the value from the current to your desired. If the value is not found, scroll to the bottom of sysctl.conf and add it. In either case, it should look like this:

vm.swappiness=40

On succeeding boots, this value will be the default. We can re-read the config file, verifying any changes:

sudo sysctl -p

Turn-offs and Turn-ons

There’s one last thing we can actually do to control swap: completely disable and enable it. This brings us back to the first section on swapon and swapoff. There are several reasons why one might want to completely turn swap off. One of those reasons is to flush whatever is in swap back to memory. As long as you actually have enough system memory for the contents of swap, you’re not going to break anything, but it might by worthwhile to ask yourself what the system might have swapped the pages out in the first place. Maybe there’s a runaway process leaking memory. Assuming everything is acting appropriately, in the long run, it is probably better to adjust swappiness as detailed above.

If you’ve decided that you want to move pages out of swap and back into memory, all you need is a couple commands, which we can combine into a single line:

sudo swapoff -a && sudo swapon -a

The -a option on both of these command stands for “auto”. To be a more specific, it tells them to operate on all swap devices located in /etc/fstab that aren’t explicitly marked at “noauto”. Don’t worry about the details for now, just know it works.

After running this command, my memory looked like this:

             total       used       free     shared    buffers     cached
Mem:          3959       3935         23          0        159       1956
-/+ buffers/cache:       1819       2139
Swap:         1953          0       1953

Notice that Swap now reports 0MB used, with a small decrease in the amount of cached and free memory. If there’s a lot of data in swap, it may take a bit of time for this to finish. Don’t panic, but instead, open another terminal and observe your memory usage:

free -ms 1

Again, -m reports usage in MB. -s 1 activates a continuous polling delay of 1 second. In other words, free -m will continue to run every 1 second until you tell it to stop with Ctrl+C. If you use this while swapoff is at work, you should see all the memory numbers adjust magically.

Last Resort

If course, if all else fails, a simple reboot will clear out not only your swap, but also anything in memory.

Wrap-up

Fortunately, Linux allows us to tweak many settings and the GNU/Linux toolset gives us a great way to handle these changes. Hopefully, this article has shed some light on how to manage that mysterious swap partition and will provide a jump point to learning more about the specific commands involved.

Part 3 will be less technical, instead focusing on a discussion about modern swap usage and how that impacts the end-user today.

Should You Use Twice the Amount of Ram as Swap Space ?

No, on servers you should not :-)
Linux and other Unix-like operating systems use the term "swap" to describe both the act of moving memory pages between RAM and disk, and the region of a disk the pages are stored on. It is common to use a whole partition of a hard disk for swapping. However, with the 2.6 Linux kernel, swap files are just as fast as swap partitions. Now, many admins (both Windows and Linux/UNIX) follow an old rule of thumb that your swap partition should be twice the size of your main system RAM. Let us say I've 32GB RAM, should I set swap space to 64 GB? Is 64 GB of swap space really required? How big should your Linux / UNIX swap space be?

Old dumb memory managers

I think the '2x swap space' rule came from Old Solaris and Windows admins. Also, earlier memory mangers were very badly designed. There were not very smart. Today, we have very smart and intelligent memory manager for both Linux and UNIX.

Nonsense rule: Twice the size of your main system RAM for Servers

According to OpenBSD FAQ:

Many people follow an old rule of thumb that your swap partition should be twice the size of your main system RAM. This rule is nonsense. On a modern system, that's a LOT of swap, most people prefer that their systems never swap. You don't want your system to ever run out of RAM+swap, but you usually would rather have enough RAM in the system so it doesn't need to swap.

Select right size for your setup

... ... ..

My friend who is a true Oracle GURU recommends something as follows for heavy duty Oracle server with fast storage such as RAID 10:

  1. Swap space == Equal RAM size (if RAM < 8GB)
  2. Swap space == 0.50 times the size of RAM (if RAM > 8GB)

Swap will just keep running servers...

Swap space will just keep operation running for a while on heavy duty servers by swapping process. You can always find out swap space utilization using any one of the following command:
cat /proc/swaps
swapon -s
free -m
top

See how to find out disk I/O and related information under Linux. In the end, you need to add more RAM, adjust software (like controlling Apache workers or using lighttpd web server to save RAM) or use some sort of load balancing.

Also, refer Linux kernel documentation for /proc/sys/vm/swappiness. With this you can fine tune swap space.

A note about Desktop and Laptop

If you are going to suspend to disk, then you need swap space more than actual RAM. For example, my laptop has 1GB RAM and swap is setup to 2GB. This only applies to Laptop or desktop but not to servers.

Kernel hackers need more swap space

If you are a kernel hacker (debugging and fixing kernel issues) and generating core dumps, you need twice the RAM swap space.

Conclusion

If Linux kernel is going to use more than 2GiB swap space at a time, all users will feel the heat. Either, you get more RAM (recommend) and move to faster storage to improve disk I/O. There are no rules, each setup and configuration is unique. Adjust values as per your requirements. Select amount of swap that is right for you.

Optimizing swap file partition [Archive] - Ubuntu Forums

April 22nd, 2008

Hi Nick,

As you have also guessed correctly, the line to change in /etc/fstab is:
# /dev/sdb7
UUID=6af97bc7-faa8-4619-a8bf-1057d5c16c63 none swap sw 0 0
which points to /dev/sdb7. We shall have it point to /dev/sda6, which is fairly easy.

1. You need to edit /etc/fstab with root permissions. First, let us back up the old file. From a terminal:
sudo cp /etc/fstab /etc/fstab.old
And then:
gksudo gedit /etc/fstab
2. You need to replace the above two lines with:
/dev/sda6 none swap sw 0 0
3. Alternatively, you can learn the UUID code for your swap partition with:
blkid
#or with:
sudo vol_id -u /dev/sda6
and write this code in:
# /dev/sda6
UUID=............................... none swap sw 0 0

4. Once you make sure that your new swap is ready to roll, you can erase the older one(/dev/sdb7), reformat it to NTFS and add it to (dev/sdb6) by either installing GParted under Ubuntu and using it or by downloading and burning the GParted Live CD:
http://gparted.sourceforge.net/livecd.php

April 22nd, 2008, 05:47 AM [This part is optional...]

5. For your information, UUID is a new way to represent partitions. Yet, since it has created problems in my previous system restore attempts, I have decided to revert to the old way of mounting partitions, which in your case, would be:
# /etc/fstab: static file system information.
#
# <file system> <mount point> <type> <options> <dump> <pass>
proc /proc proc defaults 0 0
/dev/sdb2 / ext3 defaults,errors=remount-ro 0 1
/dev/sda6 none swap sw 0 0
/dev/hda /media/cdrom0 udf,iso9660 user,noauto,exec 0 0
/dev/fd0 /media/floppy0 auto rw,user,noauto,exec 0 0

For more information on UUID's:
http://wolfger.wordpress.com/2007/08/31/ubuntu-and-uuid/
http://ubuntu-utah.ubuntuforums.org/showthread.php?t=512623


6. Regarding what swap is and the optimum swap size, it is a harddrive area used by the system to dump part of the memory temporarily, when your physical RAM space is filled by running programs and background services. Having to use the swap area is not something desirable, as the time to access the hard drive is considerably longer than the time to access RAM.

So, Ubunteros with more than enough RAM are always on the look out to minimize swap. If you have equal to or more than 1 GB RAM, unless you are running a server or many simultaneous memory intensive applications, then the system hardly ever uses your swap. If you have 512 MB RAM, swap is rarely used. On the other hand, if you have 256 MB or less RAM, you will begin to notice some hard drive activity. In this context, depending on your RAM size you may have little to no gain by transferring the swap partition of another of your hard drives this way.

There is a system setting called swappiness that regulates how much the system should depend on the swap. If you have more than enough physical RAM (say 1 GB), you may consider decreasing this value to limit the redundant use of swap. To change this value (whose default is 60 on a scale of 100) on the fly:
sysctl vm.swappiness # to view the current percentage
sudo sysctl vm.swappiness=10
and to make it permanent, edit /etc/sysctl.conf with:
gksudo gedit /etc/sysctl.conf
and add the line to the end of the file to set it automatically at boot time:
vm.swappiness=10

For more information on swap and swappiness:
http://ubuntuforums.org/showthread.php?t=727667
http://ubuntuforums.org/showpost.php?p=4537237&postcount=8
http://ubuntuforums.org/showpost.php?p=4537260&postcount=9


7. Finally, there is a terminal application "top" that lets you view the system resources (cpu usage, memory consumption per application, free and used memory, swap usage, etc.), which is the console counterpart of Gnome System Monitor. I suggest you to install a similar console process viewer "htop" instead, which has more features and exhibits more compatible output with gnome-system-monitor:
http://ubuntuforums.org/showpost.php?p=4539416&postcount=13

Chris's Wiki blog-linux-MemoryRlimits

Limiting a process's memory usage on Linux

Due to recent events I have become interested in this issue, so I have been poking around and doing some experiments. Unfortunately, while Linux has a bewildering variety of memory related per-process resource limits that you can set, most of them don't work or don't do you any good.

What you have, in theory and practice:

What I really want is something that can effectively limit a process's 'committed address space' (to use the term that /proc/meminfo and the kernel documentation on swap overcommit use). I don't care if a process wants to mmap() a 50 gigabyte file, but I care a lot if it wants 50G of anonymous, unbacked address space, because the latter is what will drive the system into out-of-memory.

Unfortunately I can imagine entirely legitimate reasons to want to mmap() huge files (especially huge sparse files) on a 64-bit machine, so any limit on the total process address space on our compute servers will have to be a soft limit.

Since the Linux kernel already tracks committed address space information for the whole system, it's possible that it would not be too much work to extend it to a per-process limit. (The likely fly in the ointment is that memory regions can be shared between processes, which complicates the accounting and raises questions about what you do when a process modifies a virtual memory region in a way that is legal for it but pushes another process sharing the VMA over its limit.)

Ask SlashdotAsk Slashdot Linux and Swap Optimization

by MikeBabcock on Friday July 09, @12:52AM EDT (#79)
(User Info) http://www.linuxsupportline.com/~pgp/

If you have 128M of ram, for now, create two 64M (or 128M if you can spare the space, but I wouldn't) swap partitions on two different drives. If you have only one drive, make it a 128M partition near the beginning ...

Partitions:

/boot
SWAP
... others ...

... this way your kernel files are always in the first 1024 cylinders but your swap is at the fastest part of older disks (newer disks use scattering to make all reads and writes the same speed).

If you use two disks, and assuming the load on both is about the same, set the priorities to be equal in /etc/fstab:

/dev/hda2 swap swap defaults,pri=1 0 0
/dev/hdc2 swap swap defaults,pri=1 0 0

This way the Linux kernel automatically uses both partitions simultaneously for faster reads and writes. If your hard drive isn't doing anything else, you'll get twice the speed out of this.

Again, with 128M of ram, you only need 128M of swap, even for most mid-level server apps. If you want to run Slashdot on your PC, you might need more :) ...

... leave some unpartitioned space in the middle of your drive to add swap to later if you need to ... (or to quickly partition and back up data to if another partition craps out).
- Michael

Jim Frost

There's been a lot of talk about the old limits and why they were there and whatnot. Here's a little technical background as well as my own rules-of-thumb on size and location.

The old "2x" rule of thumb was always silly. What was important was that it was *at least* 1x. Why? Not because the kernel dumped core into the swap (which was true, but almost nobody really cared to grope through the dump so it was usually immaterial), but because every page of physical memory had a shadow page in swap. This was an important performance optimization.

First, realize that in most cases UNIX systems had only about half as much RAM as would be needed to hold everything running in RAM (even ignoring cache). 100% overcommit was *normal*; you just couldn't afford enough memory. On such systems swap performance could make or break a system.

BSD systems (and others) would start writing pages to swap long before memory space was exhausted, but keep the pages in memory as well. If the system suddenly needed more RAM to satisfy a request, any pages already written to disk could be grabbed immediately since they could be restored from swap later if necessary. If the application decided it needed the page before that happened, it was still there. Thus you could seriously overcommit memory without having much impact on performance.

Lots of people get worried about the performance impact of the (possibly) unnecessary page writes. I've never understood that; the way these systems work is you go tell the controller to do the write, then you go back to doing what you were doing and it'll let you know when it's done. Thus unless you're maxing out your controller/disk there is almost no penalty for doing it -- and it's a huge, huge win when you need RAM fast.

Some systems (notably Windows 95) don't start writing pages until they need more RAM. Doing this makes application performance synchronous with swap performance in overcommit situations -- and as we all know, that's going to suck royally. The upside to this approach is that the performance of the machine is maximized when everything *does* fit -- you're not doing any extra work -- and disk footprint of the system is minimized (a big deal when disks were a really big part of the cost of the whole system).

Assuming that what people here have been saying is true, that Linux swap space is additive, then my guess is that it uses a pool system such that it sets a low-water mark on free pages such that it starts shadowing pages when the low-water mark is exceeded. This management system goes under the assumption that there's some number of pages you might need immediately, but that's smaller than the total available page pool. That was usually true even in the old BSD days and is most definitely true today (what with everybody's desktop having 64MB or more, what luxury).

Anyway this approach provides the best of both worlds: if you have enough memory to keep the free page pool below the low-water limit then you don't do any extra work, but if you're running in a tight or overcommit mode you usually have a bunch of pages that you can steal fast if you need them. But beware the situation where total VM use exceeds real memory plus swap minus free page pool size -- you'll start swapping synchronously as the ability of the system to shadow pages disappears.

My rule of thumb, now as always, is to set swap such that the total VM space is roughly twice what I think I'll ever need in day-to-day operations, but at least as large as RAM.

Why that big? Because sometimes you blow right through your typical VM usage and it's better to have the machine respond slowly than to have things fail. Now that disk is cheap there's not much reason not to just pick a pretty big number.

As for why make it as large as RAM, well, call it superstition. Quite a number of VM systems on the market today (most particularly Windows NT) perform quite poorly if you don't have at least as much swap as RAM, and some even limit available RAM to the size of swap if you do that (as BSD used to do). Since I don't always have the time (or sometimes even the documentation) to figure out which ones use which policy I just pick a number large enough that it can't ever be a problem.

Now, about swap location. Ideally you have a whole disk dedicated to swap, but these days disks are so large that you couldn't realistically make use of a whole disk (or even a tenth of one) for swap unless you're working with some really serious datasets.

So pretty much everyone splits up their disk.

So far in replies I've heard recommendations for putting the swap at the low cylinders, middle, and high. There are two things you trade off in position -- data transfer rate and head movement.

If you put the swap at the outside or inside of the disk you're going to be moving the heads a long way any time you need to swap a page, assuming that one or another filesystems on the disk is in regular use (typically the case). On the outside, however, you may get some of that back because data transfer rates are the highest. Putting it on the inside (low cylinder numbers) is idiotic -- they have the slowest transfer rates *and* you usually have to move the heads a lot. Modern disks are so fast that this isn't really much of a problem, but it's so easy to avoid that there's no reason to do it.

I suggest putting it somewhere in the middle, preferably between your two most active partitions. Since the swap partition is usually relatively small compared to filesystems you don't make seek time that much longer between filesystems, and you ensure that you're pretty close to the swap partition all the time.

Lastly, we've heard comments that claiming both that swapping to a file is slower than swapping to a partition, and that that's not true. It *is* true. Large files (eg a swap file) will be fragmented somewhat, forcing additional disk/head movement in some cases, and you'll also have to deal with metadata describing where on the disk the file blocks are. This eats up both in-system filesystem cache and causes additional disk activity while you load metadata that's not in the cache.

Swapping to a file is something I'd do in an emergency or if you need more swap and can't justify repartitioning or adding a new disk. If you can plan ahead of time you should avoid it, it will always be somewhat slower, although realistically speaking with today's systems you might not be able to tell.

(I feel like an old fogey. You wouldn't believe how cool I think it is that I can have pretty much as much memory and disk as I want. Why, when I was a young hacker, we had to swap our programs to paper using a pencil....)

Paging - Wikipedia, the free encyclopedia

In some older virtual memory operating systems, space in swap backing store is reserved when programs allocate memory for runtime data. OS vendors typically issue guidelines about how much swap space should be allocated. 1.5 times the installed RAM is a typical number. With a large amount of RAM, the disk space needed for the backing store can be very large. Newer versions of these operating systems attempt to solve this problem: for example, some HP-UX kernels offer a tunable swapmem_on that controls whether RAM can be used for memory reservations. In systems with sufficient RAM, this significantly reduces the needed space allocation for the backing store.

Confusion over size of swap partition - MEPISlovers Forums

G'day all.
Will repartition HDD when I replace 6.5 with 7.0 to create a separate home partition. The handbook for 7.0 recommends the size of the swap partition to be double that of your RAM, but no larger than 1Gb.
I have a recent laptop with 2Gb of RAM. My swap partition is currently set to 4Gb which was based on my reading at that time. Subsequently found the most common recommendations of swap size to be half of RAM, equal to RAM and twice size of RAM. One recommendation even stated that the swap partition is probably not necessary in these days of large RAM.

Would someone be kind enough to point me in the direction of an appropriate size for a swap partition? Many thanks in advance.
Cheers,
Miles

Linux Partition HOWTO Partitioning requirements/ How large should my swap space be?

Conventional wisdom creates swap space equal to the amount of RAM.

But keep in mind that this is just a rule of thumb. It is easily possible to create scenarios where programs have extremely large or extremely small working sets. For example, a simulation program with a large data set that is accessed in a very random fashion would have almost no noticeable locality of reference in its data segment, so its working set would be quite large.

On the other hand, a graphics program with many simultaneously opened JPEGs, all but one iconified, would have a very large data segment. But image transformations are all done on one single image, most of the memory occupied by the program is not accessed. The same is true for an editor with many editor windows where only one window is being modified at a time. These programs have - if they are designed properly - a very high locality of reference and large parts of them can be kept swapped out without too severe performance impact. A user who never never quits programs once launched would want a lot of swap space for the same reason.

Servers typically are configured with more swap space than their desktop counterparts. Even though a given amount of swap is sufficient for its operations, the server might come under transient heavy loads which cause it to page out at a high rate. Some administrators prefer this to the server crashing altogether. In these cases, swap might be several times the size of ram.

How large can my swap space be?

Currently, the maximum size of a swap partition is architecture-dependent. For i386, m68k, ARM and PowerPC, it is "officially" 2Gb. It is 128Gb on alpha, 1Gb on sparc, and 3Tb on sparc64. An opteron on the 2.6 kernel can write to a 16 Tb swap partition. For linux kernels 2.1 and earlier, the limit is 128Mb. The partition may be larger than 128 MB, but excess space is never used. If you want more than 128 MB of swap for a 2.1 and earlier kernel, you have to create multiple swap partitions (8 max). After 2.4, 32 swap areas are "officially" possible. See setting up swap for details.

Where should I put my swap space?

The short answer is anywhere is fine. However, if you are interested in extracting as much speed as possible, there are two basic strategies (other than buying more RAM):

  1. Split the swap space across multiple drives, or at least on the drive you write to least.
  2. Put each swap partition on the outer tracks.
Here are the considerations:

Summary: Put your swap on a fast disk with many heads that is not busy doing other things. If you have multiple disks: Split swap and scatter it over all your disks or even different controllers.
 

max swap size: With kernel 2.4, the limit is 64 swap spaces at a maximum of 64Gb each, although this is not reflected in the man page for mkswap. With the 64 bit opteron on the 2.6 kernel, 128 swap areas are permitted, each a whopping 16 Tb! (thanks to Peter Chubb for the calculation)
 

[Dec 03, 2007]  All about Linux swap space By Gary Sims

December 03, 2007 |  Linux.com

When your computer needs to run programs that are bigger than your available physical memory, most modern operating systems use a technique called swapping, in which chunks of memory are temporarily stored on the hard disk while other data is moved into physical memory space. Here are some techniques that may help you better manage swapping on Linux systems and get the best performance from the Linux swapping subsystem.

Linux divides its physical RAM (random access memory) into chucks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.

Swapping is necessary for two important reasons. First, when the system requires more memory than is physically available, the kernel swaps out less used pages and gives memory to the current application (process) that needs the memory immediately. Second, a significant number of the pages used by an application during its startup phase may only be used for initialization and then never used again. The system can swap out those pages and free the memory for other applications or even for the disk cache.

However, swapping does have a downside. Compared to memory, disks are very slow. Memory speeds can be measured in nanoseconds, while disks are measured in milliseconds, so accessing the disk can be tens of thousands times slower than accessing physical memory. The more swapping that occurs, the slower your system will be. Sometimes excessive swapping or thrashing occurs where a page is swapped out and then very soon swapped in and then swapped out again and so on. In such situations the system is struggling to find free memory and keep applications running at the same time. In this case only adding more RAM will help.

Linux has two forms of swap space: the swap partition and the swap file. The swap partition is an independent section of the hard disk used solely for swapping; no other files can reside there. The swap file is a special file in the filesystem that resides amongst your system and data files.

To see what swap space you have, use the command swapon -s. The output will look something like this:

Filename        Type            Size    Used    Priority
/dev/sda5       partition       859436  0       -1

Each line lists a separate swap space being used by the system. Here, the 'Type' field indicates that this swap space is a partition rather than a file, and from 'Filename' we see that it is on the disk sda5. The 'Size' is listed in kilobytes, and the 'Used' field tells us how many kilobytes of swap space has been used (in this case none). 'Priority' tells Linux which swap space to use first. One great thing about the Linux swapping subsystem is that if you mount two (or more) swap spaces (preferably on two different devices) with the same priority, Linux will interleave its swapping activity between them, which can greatly increase swapping performance.

To add an extra swap partition to your system, you first need to prepare it. Step one is to ensure that the partition is marked as a swap partition and step two is to make the swap filesystem. To check that the partition is marked for swap, run as root:

fdisk -l /dev/hdb

Replace /dev/hdb with the device of the hard disk on your system with the swap partition on it. You should see output that looks like this:

 Device Boot    Start   End     Blocks  Id      System
/dev/hdb1       2328    2434    859446  82      Linux swap / Solaris

If the partition isn't marked as swap you will need to alter it by running fdisk and using the 't' menu option. Be careful when working with partitions -- you don't want to delete important partitions by mistake or change the id of your system partition to swap by mistake. All data on a swap partition will be lost, so double-check every change you make. Also note that Solaris uses the same ID as Linux swap space for its partitions, so be careful not to kill your Solaris partitions by mistake.

Once a partition is marked as swap, you need to prepare it using the mkswap (make swap) command as root:

mkswap /dev/hdb1

If you see no errors, your swap space is ready to use. To activate it immediately, type:

swapon /dev/hdb1

You can verify that it is being used by running swapon -s. To mount the swap space automatically at boot time, you must add an entry to the /etc/fstab file, which contains a list of filesystems and swap spaces that need to be mounted at boot up. The format of each line is:

<file system>     <mount point>     <type>     <options>        <dump>    <pass>

Since swap space is a special type of filesystem, many of these parameters aren't applicable. For swap space, add:

/dev/hdb1       none    swap    sw      0       0

where /dev/hdb1 is the swap partition. It doesn't have a specific mount point, hence none. It is of type swap with options of sw, and the last two parameters aren't used so they are entered as 0.

To check that your swap space is being automatically mounted without having to reboot, you can run the swapoff -a command (which turns off all swap spaces) and then swapon -a (which mounts all swap spaces listed in the /etc/fstab file) and then check it with swapon -s.

Swap file

As well as the swap partition, Linux also supports a swap file that you can create, prepare, and mount in a fashion similar to that of a swap partition. The advantage of swap files is that you don't need to find an empty partition or repartition a disk to add additional swap space.

To create a swap file, use the dd command to create an empty file. To create a 1GB file, type:

dd if=/dev/zero of=/swapfile bs=1024 count=1048576

/swapfile is the name of the swap file, and the count of 1048576 is the size in kilobytes (i.e. 1GB).

Prepare the swap file using mkswap just as you would a partition, but this time use the name of the swap file:

mkswap /swapfile

And similarly, mount it using the swapon command: swapon /swapfile.

The /etc/fstab entry for a swap file would look like this:

/swapfile       none    swap    sw      0       0

How big should my swap space be?

It is possible to run a Linux system without a swap space, and the system will run well if you have a large amount of memory -- but if you run out of physical memory then the system will crash, as it has nothing else it can do, so it is advisable to have a swap space, especially since disk space is relatively cheap.

The key question is how much? Older versions of Unix-type operating systems (such as Sun OS and Ultrix) demanded a swap space of two to three times that of physical memory. Modern implementations (such as Linux) don't require that much, but they can use it if you configure it. A rule of thumb is as follows:

  1. For a desktop system, use a swap space of double system memory, as it will allow you to run a large number of applications (many of which may will be idle and easily swapped), making more RAM available for the active applications;
  2. For a server, have a smaller amount of swap available (say half of physical memory) so that you have some flexibility for swapping when needed, but monitor the amount of swap space used and upgrade your RAM if necessary;
  3. For older desktop machines (with say only 128MB), use as much swap space as you can spare, even up to 1GB.

The Linux 2.6 kernel added a new kernel parameter called swappiness to let administrators tweak the way Linux swaps. It is a number from 0 to 100. In essence, higher values lead to more pages being swapped, and lower values lead to more applications being kept in memory, even if they are idle. Kernel maintainer Andrew Morton has said that he runs his desktop machines with a swappiness of 100, stating that "My point is that decreasing the tendency of the kernel to swap stuff out is wrong. You really don't want hundreds of megabytes of BloatyApp's untouched memory floating about in the machine. Get it out on the disk, use the memory for something useful."

One downside to Morton's idea is that if memory is swapped out too quickly then application response time drops, because when the application's window is clicked the system has to swap the application back into memory, which will make it feel slow.

The default value for swappiness is 60. You can alter it temporarily (until you next reboot) by typing as root:

echo 50 > /proc/sys/vm/swappiness

If you want to alter it permanently then you need to change the vm.swappiness parameter in the /etc/sysctl.conf file.

Conclusion

Managing swap space is an essential aspect of system administration. With good planning and proper use swapping can provide many benefits. Don't be afraid to experiment, and always monitor your system to ensure you are getting the results you need.

Gary Sims has a degree in Business Information Systems from a British university. He worked for 10 years as a software engineer and is now a freelance Linux consultant and writer.

(4:00:00 PM)

When your computer needs to run programs that are bigger than your available physical memory, most modern operating systems use a technique called swapping, in which chunks of memory are temporarily stored on the hard disk while other data is moved into physical memory space. Here are some techniques that may help you better manage swapping on Linux systems and get the best performance from the Linux swapping subsystem.

Linux divides its physical RAM (random access memory) into chucks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.

Swapping is necessary for two important reasons. First, when the system requires more memory than is physically available, the kernel swaps out less used pages and gives memory to the current application (process) that needs the memory immediately. Second, a significant number of the pages used by an application during its startup phase may only be used for initialization and then never used again. The system can swap out those pages and free the memory for other applications or even for the disk cache.

However, swapping does have a downside. Compared to memory, disks are very slow. Memory speeds can be measured in nanoseconds, while disks are measured in milliseconds, so accessing the disk can be tens of thousands times slower than accessing physical memory. The more swapping that occurs, the slower your system will be. Sometimes excessive swapping or thrashing occurs where a page is swapped out and then very soon swapped in and then swapped out again and so on. In such situations the system is struggling to find free memory and keep applications running at the same time. In this case only adding more RAM will help.

Linux has two forms of swap space: the swap partition and the swap file. The swap partition is an independent section of the hard disk used solely for swapping; no other files can reside there. The swap file is a special file in the filesystem that resides amongst your system and data files.

To see what swap space you have, use the command swapon -s. The output will look something like this:

Filename        Type            Size    Used    Priority
/dev/sda5       partition       859436  0       -1

Each line lists a separate swap space being used by the system. Here, the 'Type' field indicates that this swap space is a partition rather than a file, and from 'Filename' we see that it is on the disk sda5. The 'Size' is listed in kilobytes, and the 'Used' field tells us how many kilobytes of swap space has been used (in this case none). 'Priority' tells Linux which swap space to use first. One great thing about the Linux swapping subsystem is that if you mount two (or more) swap spaces (preferably on two different devices) with the same priority, Linux will interleave its swapping activity between them, which can greatly increase swapping performance.

To add an extra swap partition to your system, you first need to prepare it. Step one is to ensure that the partition is marked as a swap partition and step two is to make the swap filesystem. To check that the partition is marked for swap, run as root:

fdisk -l /dev/hdb

Replace /dev/hdb with the device of the hard disk on your system with the swap partition on it. You should see output that looks like this:

 Device Boot    Start   End     Blocks  Id      System
/dev/hdb1       2328    2434    859446  82      Linux swap / Solaris

If the partition isn't marked as swap you will need to alter it by running fdisk and using the 't' menu option. Be careful when working with partitions -- you don't want to delete important partitions by mistake or change the id of your system partition to swap by mistake. All data on a swap partition will be lost, so double-check every change you make. Also note that Solaris uses the same ID as Linux swap space for its partitions, so be careful not to kill your Solaris partitions by mistake.

Once a partition is marked as swap, you need to prepare it using the mkswap (make swap) command as root:

mkswap /dev/hdb1

If you see no errors, your swap space is ready to use. To activate it immediately, type:

swapon /dev/hdb1

You can verify that it is being used by running swapon -s. To mount the swap space automatically at boot time, you must add an entry to the /etc/fstab file, which contains a list of filesystems and swap spaces that need to be mounted at boot up. The format of each line is:

<file system>     <mount point>     <type>     <options>        <dump>    <pass>

Since swap space is a special type of filesystem, many of these parameters aren't applicable. For swap space, add:

/dev/hdb1       none    swap    sw      0       0

where /dev/hdb1 is the swap partition. It doesn't have a specific mount point, hence none. It is of type swap with options of sw, and the last two parameters aren't used so they are entered as 0.

To check that your swap space is being automatically mounted without having to reboot, you can run the swapoff -a command (which turns off all swap spaces) and then swapon -a (which mounts all swap spaces listed in the /etc/fstab file) and then check it with swapon -s.

Swap file

As well as the swap partition, Linux also supports a swap file that you can create, prepare, and mount in a fashion similar to that of a swap partition. The advantage of swap files is that you don't need to find an empty partition or repartition a disk to add additional swap space.

To create a swap file, use the dd command to create an empty file. To create a 1GB file, type:

dd if=/dev/zero of=/swapfile bs=1024 count=1048576

/swapfile is the name of the swap file, and the count of 1048576 is the size in kilobytes (i.e. 1GB).

Prepare the swap file using mkswap just as you would a partition, but this time use the name of the swap file:

mkswap /swapfile

And similarly, mount it using the swapon command: swapon /swapfile.

The /etc/fstab entry for a swap file would look like this:

/swapfile       none    swap    sw      0       0

How big should my swap space be?

It is possible to run a Linux system without a swap space, and the system will run well if you have a large amount of memory -- but if you run out of physical memory then the system will crash, as it has nothing else it can do, so it is advisable to have a swap space, especially since disk space is relatively cheap.

The key question is how much? Older versions of Unix-type operating systems (such as Sun OS and Ultrix) demanded a swap space of two to three times that of physical memory. Modern implementations (such as Linux) don't require that much, but they can use it if you configure it. A rule of thumb is as follows:

  1. For a desktop system, use a swap space of double system memory, as it will allow you to run a large number of applications (many of which may will be idle and easily swapped), making more RAM available for the active applications;
  2. For a server, have a smaller amount of swap available (say half of physical memory) so that you have some flexibility for swapping when needed, but monitor the amount of swap space used and upgrade your RAM if necessary;
  3. For older desktop machines (with say only 128MB), use as much swap space as you can spare, even up to 1GB.

The Linux 2.6 kernel added a new kernel parameter called swappiness to let administrators tweak the way Linux swaps. It is a number from 0 to 100. In essence, higher values lead to more pages being swapped, and lower values lead to more applications being kept in memory, even if they are idle. Kernel maintainer Andrew Morton has said that he runs his desktop machines with a swappiness of 100, stating that "My point is that decreasing the tendency of the kernel to swap stuff out is wrong. You really don't want hundreds of megabytes of BloatyApp's untouched memory floating about in the machine. Get it out on the disk, use the memory for something useful."

One downside to Morton's idea is that if memory is swapped out too quickly then application response time drops, because when the application's window is clicked the system has to swap the application back into memory, which will make it feel slow.

The default value for swappiness is 60. You can alter it temporarily (until you next reboot) by typing as root:

echo 50 > /proc/sys/vm/swappiness

If you want to alter it permanently then you need to change the vm.swappiness parameter in the /etc/sysctl.conf file.

Conclusion

Managing swap space is an essential aspect of system administration. With good planning and proper use swapping can provide many benefits. Don't be afraid to experiment, and always monitor your system to ensure you are getting the results you need.

Gary Sims has a degree in Business Information Systems from a British university. He worked for 10 years as a software engineer and is now a freelance Linux consultant and writer.

 

Recommended Links

Linux.com -- All about Linux swap space  by Gary Sims

Talk-FAQ Linux Memory Management - Gentoo Linux Wiki

Virtual memory - Wikipedia, the free encyclopedia

Paging - Wikipedia, the free encyclopedia

The Linux Documentation Project have some documents that give in depth description of swap space issues.

The Multi Disk HOWTO which is very much about tuning in general. It is way to big to quote here so I' rather suggest the interested readers look at the following chapters listed below (straight from the Multi Disk HOWTO homepage):

Note that the swap partiotn limit in old kernels depends on the architecture of the processor, namely if it is a 32- or 64-bits processor.

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