This HOWTO describes the "new-style" RAID present in the 2.4 kernels only. It does not describe the "old-style" RAID functionality present in 2.0 and 2.2 kernels.
The home site for this HOWTO is http://unthought.net/Software-RAID.HOWTO/, where updated versions appear first. The howto is written by Jakob Østergaard based on a large number of emails between the author and Ingo Molnar (mingo@chiara.csoma.elte.hu) -- one of the RAID developers --, the linux-raid mailing list (linux-raid@vger.kernel.org) and various other people.
If you want to use the new-style RAID with 2.0 or 2.2 kernels, you should get a patch for your kernel, from http://people.redhat.com/mingo/ The standard 2.2 kernels does not have direct support for the new-style RAID described in this HOWTO. Therefore these patches are needed. The old-style RAID support in standard 2.0 and 2.2 kernels is buggy and lacks several important features present in the new-style RAID software.
Some of the information in this HOWTO may seem trivial, if you know RAID all ready. Just skip those parts.
The mandatory disclaimer:
All information herein is presented "as-is", with no warranties expressed nor implied. If you lose all your data, your job, get hit by a truck, whatever, it's not my fault, nor the developers'. Be aware, that you use the RAID software and this information at your own risk! There is no guarantee whatsoever, that any of the software, or this information, is in any way correct, nor suited for any use whatsoever. Back up all your data before experimenting with this. Better safe than sorry.
This HOWTO assumes you are using a late 2.2.x or 2.0.x kernel with a matching RAID patch and the 0.90 version of the raidtools, or that you are using linux-2.4. Both the patches and the tools can be found at http://people.redhat.com/mingo/. The RAID patch, the raidtools package, and the kernel should all match as close as possible. At times it can be necessary to use older kernels if raid patches are not available for the latest kernel.
If you use and recent GNU/Linux distribution based on the 2.4 kernel, your system most likely already has a matching version of the raidtools for your kernel.