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RAID (Redundant Array of Independent Disks) is a set of methods (or levels) for storing data on a set of disks to improve performance, reliability, or both.
RAID levels 0, 1 and 5 are most common. RAID is most commonly implemented in hardware controllers. A RAID controller appears to a host computer just as any other storage device would. Behind the hardware RAID controller is a group of disk drives. Depending on which RAID level the controller is configured for, it will store data on the disks in different ways.
The common RAID levels have the following characteristics:
The RAID levels can also be implemented in the host's software on any collection of individual disks. RAID-0 (striping) and RAID-1 (mirroring) are the simplest to implement in a host driver and there are many "software RAID" implementations.
Recovery procedures are the most difficult aspect of software RAID. Performance of software RAID may be slower than hardware RAID for a couple reasons. Software RAID levels one and higher often require more data to be transfered between hosts and storage than would be required for hardware RAID. For example, the host must make two writes to separate disks when maintaining mirrors whereas the data would be written only once to a hardware RAID device. In addition to the extra I/O to maintain parity, RAID-5 XOR calculations require extra CPU cycles.
The hardware based system manages the RAID subsystem independently from the host and presents to the host only a single disk per RAID array. This way the host doesn't have to be aware of the RAID subsystems(s).
Special and pretty complex driver is needed to implement software RAID solution. This is more error prone and less compatible then hardware based solutions, especially Fiber Channel based, but it is cheaper.
Just like any other application, software-based arrays occupy host system memory, consume CPU cycles and are operating system dependent. By contending with other applications that are running concurrently for host CPU cycles and memory, software-based arrays degrade overall server performance. Also, unlike hardware-based arrays, the performance of a software-based array is directly dependent on server CPU performance and load.
Except for the array functionality, hardware-based RAID schemes have very little in common with software-based implementations. Since the host CPU can execute user applications while the array adapter's processor simultaneously executes the array functions, the result is true hardware multi-tasking. Hardware arrays also do not occupy any host system memory, nor are they operating system dependent.
Hardware arrays are also highly fault tolerant. Since the array logic is based in hardware, software is NOT required to boot. Some software arrays, however, will fail to boot if the boot drive in the array fails. For example, an array implemented in software can only be functional when the array software has been read from the disks and is memory-resident. What happens if the server can't load the array software because the disk that contains the fault tolerant software has failed? Software-based implementations commonly require a separate boot drive, which may be included or not in the array.
Andre Molyneux's Weblog
- RAID0 is striping, in which data is spread across multiple spindles (disks). Data written to the RAID0 is broken up into chunks of a specified size (interlace value) and spread across the disks:
+--------+ +--------+ | Chunk 1| |Chunk 2 | +--------+ +--------+ | Chunk 3| |Chunk 4 | +--------+ +--------+ | Chunk 5| |Chunk 6 | +--------+ +--------+ Disk A Disk B | | +--------Stripe---------+
In this case the term RAID is a misnomer as no redundancy is gained. The address space is 'interlaced' across the disks, improving performance for I/Os bigger than the interlace/chunk size as a single I/O will spread across multiple disks. In the example above, for an interlace size of 16k, the first 16k of data would reside on Disk A, the second 16k on Disk B, the third 16k on Drive A, and so on. The interlace size needs to be chosen when the logical device is created and can't be changed later without recreating the logical device from scratch.
RAID1: RAID1 is mirroring. Every byte of data written to the mirror is duplicated on both disks:
+--------+ +--------+ | Copy A | <-> | Copy B | +--------+ +--------+ Disk A Disk B | | +--------Mirror---------+
The advantages are that you can lose either disk and the data will still be accessible, and reads can be alternated between the two disks to improve performance. The drawbacks are that you've doubled your storage costs and incurred additional overhead by having to generate two writes to physical devices for every one that's done to the logical mirror device.
Concatenation: One additional type of logical device that's involved when combining stripes and mirrors in SVM is a concatenation. There is no RAID nomencalture associated with a 'concat':
+--------+ | Disk A | +--------+ | Disk B | +--------+ | Disk C | +--------+ Concat
A concatenation aggregates several smaller physical devices into one large logical device. Unlike a stripe, the address space isn't interlaced across the underlying devices. This means there's no performance gain from using a concatenated device.
Since RAID0 improves performance, and RAID1 provides redundancy, someone came up with the idea to combine them. Fast and reliable. Two great tastes that taste great together!
When combining these two types of 'logical' devices there's a choice to be made -- do you mirror two stripes, or do you stripe across multiple mirrors? There are pros and cons to each approach:
- RAID 0+1: In RAID 0+1, the stripes are created first, then are mirrored together. Logically, the resulting device looks like:
+------------------------------------+ +------------------------------------+ | +--------+ +--------+ +--------+ | | +--------+ +--------+ +--------+ | | | Chunk 1| |Chunk 2 | |Chunk 3 | | | | Chunk 1| |Chunk 2 | |Chunk 3 | | | +--------+ +--------+ +--------+ | | +--------+ +--------+ +--------+ | | | Chunk 4| |Chunk 5 | |Chunk 6 | | | | Chunk 4| |Chunk 5 | |Chunk 6 | | | +--------+ +--------+ +--------+ |<--->| +--------+ +--------+ +--------+ | | | Chunk 7| |Chunk 8 | |Chunk 9 | | | | Chunk 7| |Chunk 8 | |Chunk 9 | | | +--------+ +--------+ +--------+ | | +--------+ +--------+ +--------+ | | Disk A Disk B Disk C | | Disk D Disk E Disk F | +------------------------------------+ +------------------------------------+ Stripe 1 Stripe 2 | | +-----------------------------------Mirror--------------------------------------+
Advantage: Simple administrative model. Issue one command to create the first stripe, a second command to create the second stripe, and a third command to mirror them. Three commands and you're done, regardless of the number of disks in the configuration.
Disadvantage: An error on any one of the disks kills redundancy for all disks. For instance, a failure on Disk B above 'breaks' the Stripe 1 side of the mirror. As a result, should disk D, E, or F fail as well, the entire mirror becomes unusable.
RAID 1+0: In RAID 1+0 the opposite approach is taken. The disks are mirrored first, then the mirrors are combined together into a stripe:
+-----------------+ +-----------------+ +-----------------+ | Chunk 1 | | Chunk 2 | | Chunk 3 | +-----------------+ +-----------------+ +-----------------+ | Chunk 4 | | Chunk 5 | | Chunk 6 | +-----------------+ +-----------------+ +-----------------+ | Chunk 7 | | Chunk 8 | | Chunk 9 | +-----------------+ +-----------------+ +-----------------+ Disk A <---> Disk B Disk C <---> Disk D Disk E <---> Disk F Mirror 1 Mirror 2 Mirror 3 | | +-----------------------------------------------------------+ Stripe
Advantage: A failure on one disk only impacts redundancy for the chunks of the stripe that are located on that disk. For instance, a failure on Disk B above only loses redundancy for every third chunk (1, 4, 7, etc.) Redundancy for the other stripe chunks is unaffected, so a second disk failure could be tolerated as long as the second failure wasn't on Disk A.
Disadvantage: More complicated from an administrative standpoint. The administrator needs to issue one creation command per mirror, then a command to stripe across the mirrors. The six-disk example above would require four commands to create, while a twelve disk configuration would require seven commands.
[Nov 02, 2019] Raid-5 is obsolete if you use large drives , such as 2TB or 3TB disks. You should instead use raid-6 ( two disks can fail)
"... RAID5 can survive a single drive failure. However, once you replace that drive, it has to be initialized. Depending on the controller and other things, this can take anywhere from 5-18 hours. During this time, all drives will be in constant use to re-create the failed drive. It is during this time that people worry that the rebuild would cause another drive near death to die, causing a complete array failure. ..."
"... If during a rebuild one of the remaining disks experiences BER, your rebuild stops and you may have headaches recovering from such a situation, depending on controller design and user interaction. ..."
"... RAID5 + a GOOD backup is something to consider, though. ..."
"... Raid-5 is obsolete if you use large drives , such as 2TB or 3TB disks. You should instead use raid-6 ..."
"... RAID 6 offers more redundancy than RAID 5 (which is absolutely essential, RAID 5 is a walking disaster) at the cost of multiple parity writes per data write. This means the performance will be typically worse (although it's not theoretically much worse, since the parity operations are in parallel). ..."
Oct 03, 2019 | hardforum.com
RAID5 can survive a single drive failure. However, once you replace that drive, it has to be initialized. Depending on the controller and other things, this can take anywhere from 5-18 hours. During this time, all drives will be in constant use to re-create the failed drive. It is during this time that people worry that the rebuild would cause another drive near death to die, causing a complete array failure.
This isn't the only danger. The problem with 2TB disks, especially if they are not 4K sector disks, is that they have relative high BER rate for their capacity, so the likelihood of BER actually occurring and translating into an unreadable sector is something to worry about.
If during a rebuild one of the remaining disks experiences BER, your rebuild stops and you may have headaches recovering from such a situation, depending on controller design and user interaction.
So i would say with modern high-BER drives you should say:
- RAID5: 0 complete disk failures, BER covered
- RAID6: 1 complete disk failure, BER covered
So essentially you'll lose one parity disk alone for the BER issue. Not everyone will agree with my analysis, but considering RAID5 with today's high-capacity drives 'safe' is open for debate.
RAID5 + a GOOD backup is something to consider, though.
- So you're saying BER is the error count that 'escapes' the ECC correction? I do not believe that is correct, but i'm open to good arguments or links.
As i understand, the BER is what prompt bad sectors, where the number of errors exceed that of the ECC error correcting ability; and you will have an unrecoverable sector (Current Pending Sector in SMART output).
Also these links are interesting in this context:
http://blog.econtech.selfip.org/200...s-not-fully-readable-a-lawsuit-in-the-making/The short story first: Your consumer level 1TB SATA drive has a 44% chance that it can be completely read without any error. If you run a RAID setup, this is really bad news because it may prevent rebuilding an array in the case of disk failure, making your RAID not so Redundant. Click to expand...Not sure on the numbers the article comes up with, though.
Also this one is interesting:
http://lefthandnetworks.typepad.com/virtual_view/2008/02/what-does-data.htmlBER simply means that while reading your data from the disk drive you will get an average of one non-recoverable error in so many bits read, as specified by the manufacturer. Click to expand...Rebuilding the data on a replacement drive with most RAID algorithms requires that all the other data on the other drives be pristine and error free. If there is a single error in a single sector, then the data for the corresponding sector on the replacement drive cannot be reconstructed, and therefore the RAID rebuild fails and data is lost. The frequency of this disastrous occurrence is derived from the BER. Simple calculations will show that the chance of data loss due to BER is much greater than all other reasons combined. Click to expand...These links do suggest that BER works to produce un-recoverable sectors, and not 'escape' them as 'undetected' bad sectors, if i understood you correctly.
- parityOCP said: ↑That's guy's a bit of a scaremonger to be honest. He may have a point with consumer drives, but the article is sensationalised to a certain degree. However, there are still a few outfits that won't go past 500GB/drive in an array (even with enterprise drives), simply to reduce the failure window during a rebuild. Click to expand...Why is he a scaremonger? He is correct. Have you read his article? In fact, he has copied his argument from Adam Leventhal(?) that was one of the ZFS developers I believe.
Adam's argument goes likes this:
Disks are getting larger all the time, in fact, the storage increases exponentially. At the same time, the bandwidth is increasing not that fast - we are still at 100MB/sek even after decades. So, bandwidth has increased maybe 20x after decades. While storage has increased from 10MB to 3TB = 300.000 times.
The trend is clear. In the future when we have 10TB drives, they will not be much faster than today. This means, to repair an raid with 3TB disks today, will take several days, maybe even one week. With 10TB drives, it will take several weeks, maybe a month.
Repairing a raid stresses the other disks much, which means they can break too. Experienced sysadmins reports that this happens quite often during a repair. Maybe because those disks come from the same batch, they have the same weakness. Some sysadmins therefore mix disks from different vendors and batches.
Hence, I would not want to run a raid with 3TB disks and only use raid-5. During those days, if only another disk crashes you have lost all your data.
Hence, that article is correct, and he is not a scaremonger. Raid-5 is obsolete if you use large drives, such as 2TB or 3TB disks. You should instead use raid-6 (two disks can fail). That is the conclusion of the article: use raid-6 with large disks, forget raid-5. This is true, and not scaremongery.
In fact, ZFS has therefore something called raidz3 - which means that three disks can fail without problems. To the OT: no raid-5 is not safe. Neither is raid-6, because neither of them can not always repair nor detect corrupted data. There are cases when they dont even notice that you got corrupted bits. See my other thread for more information about this. That is the reason people are switching to ZFS - which always CAN detect and repair those corrupted bits. I suggest, sell your hardware raid card, and use ZFS which requires no hardware. ZFS just uses JBOD.
Here are research papers on raid-5, raid-6 and ZFS and corruption:
Is RAID5 safe with Five 2TB Hard Drives ? | [H]ard|Forum Your browser indicates if you've visited this link
- brutalizer said: ↑The trend is clear. In the future when we have 10TB drives, they will not be much faster than today. This means, to repair an raid with 3TB disks today, will take several days, maybe even one week. With 10TB drives, it will take several weeks, maybe a month. Click to expand...While I agree with the general claim that the larger HDDs (1.5, 2, 3TBs) are best used in RAID 6, your claim about rebuild times is way off.
I think it is not unreasonable to assume that the 10TB drives will be able to read and write at 200 MB/s or more. We already have 2TB drives with 150MB/s sequential speeds, so 200 MB/s is actually a conservative estimate.
10e12/200e6 = 50000 secs = 13.9 hours. Even if there is 100% overhead (half the throughput), that is less than 28 hours to do the rebuild. It is a long time, but it is no where near a month! Try to back your claims in reality.
And you have again made the false claim that "ZFS - which always CAN detect and repair those corrupted bits". ZFS can usually detect corrupted bits, and can usually correct them if you have duplication or parity, but nothing can always detect and repair. ZFS is safer than many alternatives, but nothing is perfectly safe. Corruption can and has happened with ZFS, and it will happen again.
Hence, that article is correct, and he is not a scaremonger. Raid-5 is obsolete if you use large drives , such as 2TB or 3TB disks. You should instead use raid-6 ( two disks can fail). That is the conclusion of the article: use raid-6 with large disks, forget raid-5 . This is true, and not scaremongery.
RAID 5 Data Recovery How to Rebuild a Failed RAID 5 - YouTubeRAID 5 vs RAID 10: Recommended RAID For Safety and ... Your browser indicates if you've visited this link
RAID 6 offers more redundancy than RAID 5 (which is absolutely essential, RAID 5 is a walking disaster) at the cost of multiple parity writes per data write. This means the performance will be typically worse (although it's not theoretically much worse, since the parity operations are in parallel).
So, does SVM do RAID 0+1 or RAID 1+0? The answer is, "Yes." So it gives you a choice between the two? The answer is "No."
Obviously further explanation is necessary...
In SVM, mirror devices cannot be created from "bare" disks. You are required to create the mirror on top of another type of SVM metadevice, known as a concat/stripe*. SVM combines concatenations and stripes into a single metadevice type, in which one or more stripes are concatenated together. When used to build a mirror these concat/stripe logical devices are known as submirrors. If you want to expand the size of a mirror device you can do so by concatenating additional stripe(s) onto the concat/stripe devices that are serving as submirrors.
So, in SVM, you are always required to set up a stripe (concat/stripe) in order to create a mirror. On the surface this makes it appear that SVM does RAID 0+1. However, once you understand a bit about the SVM mirror code, you'll find RAID 1+0 lurking under the covers.
SVM mirrors are logically divided up into regions. The state of each mirror region is recorded in state database replicas* stored on disk. By individually recording the state of each region in the mirror, SVM can be smart about how it performs a resync. Following a disk failure or an unusual event (e.g. a power failure occurs after the first side of a mirror has been written to but before the matching write to the second side can be accomplished), SVM can determine which regions are out-of-sync and only synchronize them, not the entire mirror. This is known as an optimized resync.
The optimized resync mechanisms allow SVM to gain the redundancy benefits of RAID 1+0 while keeping the administrative benefits of RAID 0+1. If one of the drives in a concat/stripe device fails, only those mirror regions that correspond to data stored on the failed drive will lose redundancy. The SVM mirror code understands the layout of the concat/stripe submirrors and can therefore determine which resync regions reside on which underlying devices. For all regions of the mirror not affected by the failure, SVM will continue to provide redundancy, so a second disk failure won't necessarily prove fatal.
So, in a nutshell, SVM provides a RAID 0+1 style administrative interface but effectively implements RAID 1+0 functionality. Administrators get the best of each type, the relatively simple administration of RAID 0+1 plus the greater resilience of RAID 1+0 in the case of multiple device failures.
* concat/stripe logical devices (metadevices)
The following example shows a concat/stripe metadevice that's serving as a submirror to a mirror metadevice. Note that the metadevice is a concatenation of three separate stripes:
- Stripe 0 is a 1-way stripe (so not really striped at all) on disk slice c1t11d0s0.
- Stripe 1 is a 1-way stripe on disk slice c1t12d0s0.
- Stripe 2 is a 2-way stripe with an interlace size of 32 blocks on disk slices c1t13d0s1 and c1t14d0s2.
d1: Submirror of d0 State: Okay Size: 78003 blocks (38 MB) Stripe 0: Device Start Block Dbase State Reloc Hot Spare c1t11d0s0 0 No Okay Yes Stripe 1: Device Start Block Dbase State Reloc Hot Spare c1t12d0s0 0 No Okay Yes Stripe 2: (interlace: 32 blocks) Device Start Block Dbase State Reloc Hot Spare c1t13d0s1 0 No Okay Yes c1t14d0s2 0 No Okay Yes
** State database replicas
SVM stores configuration and state information in a 'state database' in memory. Copies of this state database are stored on disk, where they are referred to as state database replicas. The primary purpose of the state database replicas is to provide non-volatile copies of the state database so that the SVM configuration is persistant across reboots. A secondary purpose of the replicas is to provide a 'scratch pad' to keep track of mirror region states.
RAID - Wikipedia, the free encyclopedia
How To Set Up Software RAID1 On A Running LVM System (Incl. GRUB Configuration) (Fedora 8) HowtoForge - Linux Howtos and Tutorials
Redundant Arrays of Independent Disks - Computerworld
Sys Admin v12, i06 Introduction to RAID
Raid Recovery Comparison Chart and Raid Types
Min. Num of Drives
2 Data striping without redundancy Highest performance No data protection; One drive fails, all data is lost
2 Disk mirroring Very high performance; Very high data protection; Very minimal penalty on write performance High redundancy cost overhead; Because all data is duplicated, twice the storage capacity is required
Not Used In LAN No practical use Previously used for RAM error environments correction (known as Hamming Code ) and in disk drives before the use of embedded error correction No practical use; Same performance can be achieved by RAID 3 at lower cost
3 Byte-level data striping with dedicated parity drive Excellent performance for large, sequential data requests Not well-suited for transaction-oriented network applications; Single parity drive does not support multiple, simultaneous read and write requests
3 (not widely used Block-level data striping with dedicated parity drive Data striping supports multiple simultaneous read requests Write requests suffer from same single parity-drive bottleneck as RAID 3; RAID 5 offers equal data protection and better performance at same cost
3 Block-level data striping with distributed parity Best cost/performance for transaction-oriented networks; Very high performance, very high data protection; Supports multiple simultaneous reads and writes; Can also be optimized for large, sequential requests Write performance is slower than RAID 0 or RAID 1
4 Combination of RAID 0 (data striping) and RAID 1 (mirroring) Highest performance, highest data protection (can tolerate multiple drive failures) High redundancy cost overhead; Because all data is duplicated, twice the storage capacity is required; Requires minimum of four drives
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