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Software or Hardware RAID under Linux

Posted on 2000-03-28
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Last Modified: 2010-04-20
Hi , Looking at a Linux Software RAID1 solution for a PC based web server:

 - What are disadvantages when compared against HW RAID solution.
 - Do I still need hot swap disks ?
 - Which is best Linux kernel & distribution to use?. Do I need any patches ?
 - any web sites/newsgroups for guidance ?

Cheers.

Steve T

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Question by:steague
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kiffney earned 300 total points
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The hardware raid controllers nowadays come with battery-backed-up RAM so in the event of a sudden outage, any cached writes to disk can be completed.  And generally if your applications are write intensive (web servers usually aren't) the hardware raid solution will be faster.  It's more expensive, though - hardware raid is almost exclusively for SCSI disks which do cost more.  And the hardware controller is less likely to fail because of misconfiguration, etc - you set it up in the BIOS, you're done.

That said, Linux's software RAID is pretty good, works with cheaper IDE disks, and if you have your whole system on a UPS and run the ups monitoring daemon, you should be OK. If this is a high-volume server, you will overall have better performance with SCSI disks and a hardware raid controller as that frees more of the processor (and ram) for dishing out your web pages, which is the whole point.  If you do go software raid, be sure you read every darn HOWTO over and over until you really understand them - I see some pretty sad messages on the forums from people who've misconfigured their software raid and screwed up their filesystems.

Hot swap only works with hardware raid controllers that support it; it will NOT work with software raid.  I personally do like to have the hot swap disks right there just for reliability's sake.   And if you're running raid5, when one disk does fail, everything slows down till you get the replacement disk in there and finish rebuilding.

The latest 2.2.14 kernel should hold you fine.  I use RedHat which compiles RAID support in by default (at least starting with 6.0) - you need to have a kernel with RAID compiled in for the software raid deal to work. but SUSE and Caldera have many fans.  There usually is a RAID patch for a particular kernel version, but if you get redhat, the patches will already be applied.  If you do roll your own setup (not recommended if you're new to this) make sure you have the version of raidtools that goes with your raid patch version. The only other patches I would think of is if you get an AMI Megaraid hardware raid controller - it's got some problems that I don't think are fixed yet.  Buslogic is a pricey controller but, in my opinion, it is the best.

Keep in mind that booting from raid can be funky.  It's probably best to have a non-raid boot partition (perhaps a little old IDE drive on its own controller), or if you are using Raid-1 (as you said you were) you can boot from the raid drive with a patch to LILO.  I think this is incorporated into the redhat 6.1 distribution.

The writer of the software raid howto keeps the current version at

http://ostenfeld.dk/~jakob/Software-RAID.HOWTO/Software-RAID.HOWTO-3.html#toc3
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by:mixerfix122699
ID: 2667103
I would further add that hardware raid is divided into two basic classes - PCI-SCSI and SCSI-SCSI.

The PCI-SCSI raid is an actual raid controller (e.g. Mylex, ICP-Vortex, etc) sitting in a PCI bus and requiring proper drivers. At least the Vortex _does_ allow booting off the controller (the RAID being the only controller in the system).

SCSI-SCSI RAID is much more expensive, in general, and is hooked to a normal SCSI controller. However, many more controller types are supported and quite wierd configurations may be achieved with such beasts (some Mylex subtypes fall into this category).
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