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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 Technicalities

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.

Be sure that the bus(ses) to the drives are fast enough. You shouldn't have 14 UW-SCSI drives on one UW bus, if each drive can give 10 MB/s and the bus can only sustain 40 MB/s. Also, you should only have one device per IDE bus. Running disks as master/slave is horrible for performance. IDE is really bad at accessing more that one drive per bus. Of Course, all newer motherboards have two IDE busses, so you can set up two disks in RAID without buying more controllers.

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 Terms

The word ``RAID'' means ``Linux Software RAID''. This HOWTO does not treat any aspects of Hardware RAID.

When describing 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.

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''.

2.3 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. Then come back when you are having problems :)

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 survive a disk crash. 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.

2.4 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 the 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, except for when the array is reconstructing (eg. after a new disk is inserted into a degraded array). When 2.4 comes out this is an issue that will most likely get addressed fairly quickly, but until then, you should stress-test the system yourself until you are either satisfied with the stability or conclude that you won't be swapping on RAID.

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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