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2. Why RAID ?

There can be many good reasons for using RAID. A few are; the ability to combine several physical disks into one larger ``virtual'' device, performance improvements, and redundancy.

2.1 Device and filesystem support

Linux RAID can work on most block devices. It doesn't matter whether you use IDE or SCSI devices, or a mixture. Some people have also used the Network Block Device (NBD) with more or less success.

Since a Linux Software RAID device is itself a block device, the above implies that you can actually create a RAID of other RAID devices. This in turn makes it possible to support RAID-10 (RAID-0 of multiple RAID-1 devices), simply by using the RAID-0 and RAID-1 functionality together. Other more exotic configurations, such a RAID-5 over RAID-5 "matrix" configurations are equally supported.

The RAID layer has absolutely nothing to do with the filesystem layer. You can put any filesystem on a RAID device, just like any other block device.

2.2 Performance

Often RAID is employed as a solution to performance problems. While RAID can indeed often be the solution you are looking for, it is not a silver bullet. There can be many reasons for performance problems, and RAID is only the solution to a few of them.

In the description of The RAID levels, there will be a mention of the performance characteristics of each level.

2.3 Terms

The word ``RAID'' means ``Linux Software RAID''. This HOWTO does not treat any aspects of Hardware RAID. Furthermore, it does not treat any aspects of Software RAID in other operating system kernels.

When describing RAID setups, it is useful to refer to the number of disks and their sizes. At all times the letter N is used to denote the number of active disks in the array (not counting spare-disks). The letter S is the size of the smallest drive in the array, unless otherwise mentioned. The letter P is used as the performance of one disk in the array, in MB/s. When used, we assume that the disks are equally fast, which may not always be true in real-world scenarios.

Note that the words ``device'' and ``disk'' are supposed to mean about the same thing. Usually the devices that are used to build a RAID device are partitions on disks, not necessarily entire disks. But combining several partitions on one disk usually does not make sense, so the words devices and disks just mean ``partitions on different disks''.

2.4 The RAID levels

Here's a short description of what is supported in the Linux RAID patches. Some of this information is absolutely basic RAID info, but I've added a few notices about what's special in the Linux implementation of the levels. Just skip this section if you know RAID.

The current RAID patches for Linux supports the following levels:

Spare disks

Spare disks are disks that do not take part in the RAID set until one of the active disks fail. When a device failure is detected, that device is marked as ``bad'' and reconstruction is immediately started on the first spare-disk available.

Thus, spare disks add a nice extra safety to especially RAID-5 systems that perhaps are hard to get to (physically). One can allow the system to run for some time, with a faulty device, since all redundancy is preserved by means of the spare disk.

You cannot be sure that your system will keep running after a disk crash though. The RAID layer should handle device failures just fine, but SCSI drivers could be broken on error handling, or the IDE chipset could lock up, or a lot of other things could happen.

Also, once reconstruction to a hot-spare begins, the RAID layer will start reading from all the other disks to re-create the redundant information. If multiple disks have built up bad blocks over time, the reconstruction itself can actually trigger a failure on one of the "good" disks. This will lead to a complete RAID failure. If you do frequent backups of the entire filesystem on the RAID array, then it is highly unlikely that you would ever get in this situation - this is another very good reason for taking frequent backups. Remember, RAID is not a substitute for backups.

2.5 Swapping on RAID

There's no reason to use RAID for swap performance reasons. The kernel itself can stripe swapping on several devices, if you just give them the same priority in the fstab file.

A nice fstab looks like:

/dev/sda2       swap           swap    defaults,pri=1   0 0
/dev/sdb2       swap           swap    defaults,pri=1   0 0
/dev/sdc2       swap           swap    defaults,pri=1   0 0
/dev/sdd2       swap           swap    defaults,pri=1   0 0
/dev/sde2       swap           swap    defaults,pri=1   0 0
/dev/sdf2       swap           swap    defaults,pri=1   0 0
/dev/sdg2       swap           swap    defaults,pri=1   0 0
This setup lets the machine swap in parallel on seven SCSI devices. No need for RAID, since this has been a kernel feature for a long time.

Another reason to use RAID for swap is high availability. If you set up a system to boot on eg. a RAID-1 device, the system should be able to survive a disk crash. But if the system has been swapping on the now faulty device, you will for sure be going down. Swapping on a RAID-1 device would solve this problem.

There has been a lot of discussion about whether swap was stable on RAID devices. This is a continuing debate, because it depends highly on other aspects of the kernel as well. As of this writing, it seems that swapping on RAID should be perfectly stable, you should however stress-test the system yourself until you are satisfied with the stability.

You can set up RAID in a swap file on a filesystem on your RAID device, or you can set up a RAID device as a swap partition, as you see fit. As usual, the RAID device is just a block device.


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