Softpanorama
(slightly skeptical) Open Source Software Educational Society

May the source be with you, but remember the KISS principle ;-)

Softpanorama Search

Linux Logical Volume Snapshots

News Snapshots Recommended Links LVM Disk Repartitioning
AIX snapshots Linux snapshots Solaris Snapshots Humor Etc

Snapshots are ``frozen'' images of a logical volume which can be used to back up the file system of some files on it (for example a database). A user command is used to initiate a snapshot at which point the current state of the LV is preserved and can be accessed through a special device node corresponding to the active LV's device node. A backup process uses the read-only ``snapped'' LV which does not change. The file system or database can continue running while the backup takes place.

They are now available on all major OSes:

A snapshot LV generally requires only a fraction of the space of the original LV because only changed data needs new space. The actual size should be tuned according to the frequency of writes to the live volume and the life span of the snapshot volume. If write activity changes to the original volume after the snapshot has been taken, the snapshot size can be changed at run time. This is especially useful if too little space was allocated for the snapshot volume.

A snapshot is created as a new LV in the same volume group as the target LV. The volume group must have free space available to allocate the snapshot LV. The name of the snapshot LV can be assigned by the user just as the name of any new LV.

Snapshots are read/write by default. Read/write snapshots work like read-only snapshots, with the additional feature that if data is written to the snapshot, that block is marked in the exception table as used, and never gets copied from the original volume. This opens up many new possibilities that were not possible with LVM1's read-only snapshots. One example is to snapshot a volume, mount the snapshot, and try an experimental program that change files on that volume. If you don't like what it did, you can unmount the snapshot, remove it, and mount the original filesystem in its place. It is also useful for creating volumes for use with Xen. You can create a disk image, then snapshot it and modify the snapshot for a particular domU instance. You can then create another snapshot of the original volume, and modify that one for a different domU instance. Since the only storage used by a snapshot is blocks that were changed on the origin or the snapshot, the majority of the volume is shared by the domU's.

Notes:
  • This is a Spartan WHYFF (We Help You For Free) site written by people for whom English is not a native language. Some amount of grammar and spelling errors should be expected.
  • The site contain some broken links as it develops like a living tree... Please try to use Google, Open directory, etc. to find a replacement link (see HOWTO search the WEB for details). We would appreciate if you can mail us a correct link.
Google Search
Open directory
Research Index

Old News ;-)

LVM HOWTO

Following on from the previous example we now want to use the extra space in the "ops" volume group to make a database backup every evening. To ensure that the data that goes onto the tape is consistent we use an LVM snapshot logical volume.

A snapshot volume is a special type of volume that presents all the data that was in the volume at the time the snapshot was created. For a more detailed description, see Section 3.8, Snapshots. This means we can back up that volume without having to worry about data being changed while the backup is going on, and we don't have to take the database volume offline while the backup is taking place.

 
Note

In LVM1, this type of volume was read-only, but LVM2 creates read/write snapshots by default.

13.4.1. Create the snapshot volume

There is a little over 500 Megabytes of free space in the "ops" volume group, so we will use all of it to allocate space for the snapshot logical volume. A snapshot volume can be as large or a small as you like but it must be large enough to hold all the changes that are likely to happen to the original volume during the lifetime of the snapshot. So here, allowing 500 megabytes of changes to the database volume which should be plenty. 

# lvcreate -L592M -s -n dbbackup /dev/ops/databases 
lvcreate -- WARNING: the snapshot must be disabled if it gets full
lvcreate -- INFO: using default snapshot chunk size of 64 KB for "/dev/ops/dbbackup"
lvcreate -- doing automatic backup of "ops"
lvcreate -- logical volume "/dev/ops/dbbackup" successfully created
 
  Full snapshot are automatically disabled
 

If the snapshot logical volume becomes full it will be dropped (become unusable) so it is vitally important to allocate enough space. The amount of space necessary is dependent on the usage of the snapshot, so there is no set recipe to follow for this. If the snapshot size equals the origin size, it will never overflow.

13.4.2. Mount the snapshot volume

We can now create a mount-point and mount the volume 

# mkdir /mnt/ops/dbbackup
# mount /dev/ops/dbbackup /mnt/ops/dbbackup
mount: block device /dev/ops/dbbackup is write-protected, mounting read-only

If you are using XFS as the filesystem you will need to add the nouuid option to the mount command: 

# mount /dev/ops/dbbackup /mnt/ops/dbbackup -onouuid,ro

13.4.3. Do the backup

I assume you will have a more sophisticated backup strategy than this! 

# tar -cf /dev/rmt0 /mnt/ops/dbbackup
tar: Removing leading `/' from member names

13.4.4. Remove the snapshot

When the backup has finished you can now unmount the volume and remove it from the system. You should remove snapshot volume when you have finished with them because they take a copy of all data written to the original volume and this can hurt performance. 

# umount /mnt/ops/dbbackup
# lvremove /dev/ops/dbbackup 
lvremove -- do you really want to remove "/dev/ops/dbbackup"? [y/n]: y
lvremove -- doing automatic backup of volume group "ops"
              lvremove -- logical volume "/dev/ops/dbbackup" successfully removed

Consistent backup with Linux Logical Volume Manager (LVM) snapshots

LVM is an implementation of a logical volume manager for the Linux kernel. The biggest advantage is that LVM provides the ability to make a snapshot of any logical volume.

In a production environment many users access the same file (file is open) or database. Suppose you start backup process when a file is open, you will not get correct or updated copy of file i.e. inconsistent backup (see accurate definition of inconsistent backup).

Read-only partition - to avoid inconsistent backup

You need to mount partition as read only, so that no one can make changes to file and make a backup:

# umount /home
# mount -o ro /home
# tar -cvf /dev/st0 /home
# umount /home
# mount -o rw /home

As you see, the draw back is service remains unavailable during backup time to end users as partition mounted as read-only. If you are using database then shutdown database server and make a backup.

This solution will only work if you have created the partition with LVM. A snapshot volume is a special type of volume that presents all the data that was in the volume at the time the snapshot was created. This means you can back up that volume without having to worry about data being changed while the backup is going on, and you don't have to take the database volume offline while the backup is taking place.

# lvcreate -L1000M -s -n dbbackup /dev/ops/databases

Output: 

lvcreate -- WARNING: the snapshot must be disabled if it gets full
lvcreate -- INFO: using default snapshot chunk size of 64 KB for "/dev/ops/dbbackup"
lvcreate -- doing automatic backup of "ops"
lvcreate -- logical volume "/dev/ops/dbbackup" successfully created

Create a mount-point and mount the volume:

# mkdir /mnt/ops/dbbackup
# mount /dev/ops/dbbackup /mnt/ops/dbbackup

Output:

mount: block device /dev/ops/dbbackup is write-protected, mounting read-only

Do the backup

# tar -cf /dev/st0 /mnt/ops/dbbackup

Now remove it:

# umount /mnt/ops/dbbackup
# lvremove /dev/ops/databases

Please note that LVM snapshots cannot be used with non-LVM filesystems i.e. you need LVM partitions. You can also use third party commercial proprietary (see below for discussion) or GPL backup solutions/software.

redhat.com Red Hat Support

Logical Volume Manager (LVM) provides the ability to take a snapshot of any logical volume for the purpose of obtaining a backup of a partition in a consistent state. As applications may access files or databases on a partition during a backup some files may be backed up in one state, while later files are backed up after an update has been made, leading to an inconsistent backup.

Traditionally the solution has been to mount the partition read-only, apply table-level write locks to databases or shut down the database engine etc.; all measures which adversely impact availability (but not as much as data loss without a backup will). With LVM snapshots it is possible to obtain a consistent backup without compromising availability.

Please note that this information is only valid for partitions that have been created using LVM. LVM snapshots cannot be used with non-LVM filesystems.

The LVM snapshot works by logging the changes to the filesystem to the snapshot partition, rather than mirroring the partition. Thus when you create a snapshot partition you do not need to use space equal to the size of the partition that you are taking a snapshot of, but rather the amount of changes that it will undergo during the lifetime of the snapshot. This is a function of both how much data is being written to the partition and also how long you intend keeping the LVM snapshot. The longer you leave it, the more changes there are likely to be on the file system and the more the snapshot partition will fill up with change information. The higher the rate of change on the partition the shorter the lifespan of the snapshot. If the amount of changes on the LVM partition exceed the size of the snapshot then the snapshot is released.

Now we will show an example of how to make an LVM snapshot. Here we create a logical volume of 500MB to use to take a snapshot. This will allow 500MB of changes on the volume we are taking a snapshot of during the lifetime of the snapshot.

The following command will create /dev/ops/dbbackup as a snapshot of /dev/ops/databases. 

# lvcreate -L500M -s -n dbbackup /dev/ops/databases 
lvcreate -- WARNING: the snapshot must be disabled if it gets full
lvcreate -- INFO: using default snapshot chunk size of 64 KB for "/dev/ops/dbbackup"
lvcreate -- doing automatic backup of "ops"
lvcreate -- logical volume "/dev/ops/dbbackup" successfully created

Now we create the mount point and mount the snapshot.

# mkdir /mnt/ops/dbbackup
# mount /dev/ops/dbbackup /mnt/ops/dbbackup
mount: block device /dev/ops/dbbackup is write-protected, mounting read-only

After performing the backup of the snapshot partition we release the snapshot. The snapshot will be automatically released when it fills up, but maintaining incurs a system overhead in the meantime. 

 
# umount /mnt/ops/dbbackup
# lvremove /dev/ops/dbbackup 
lvremove -- do you really want to remove "/dev/ops/dbbackup"? [y/n]: y
lvremove -- doing automatic backup of volume group "ops"
lvremove -- logical volume "/dev/ops/dbbackup" successfully removed

For more information about using LVM you can consult the Linux Documentation Project's HOW-TO on the subject at http://tldp.org/HOWTO/LVM-HOWTO/.

Recommended Links

Copyright © 1996-2009 by Dr. Nikolai Bezroukov. www.softpanorama.org was created as a service to the UN Sustainable Development Networking Programme (SDNP) in the author free time. Submit comments This document is an industrial compilation designed and created exclusively for educational use and is placed under the copyright of the Open Content License(OPL). Site uses AdSense so you need to be aware of Google privacy policy. Original materials copyright belong to respective owners. Quotes are made for educational purposes only in compliance with the fair use doctrine.

Disclaimer:

Last modified: April 21, 2009