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Re: [K12OSN] Linux "Software RAID"

Carl Keil wrote:
Hey Folks,

I hear people extolling the virtues of "software RAID" on the list a lot. I'm finally setting up a production server in a school and I have enough disks to play with to do RAID. I'm leaning towards RAID 5. Anyway, when people say "software RAID" do they mean just setting up a RAID in LVM Manager? Or is the mdadm command the simpler, more robust, preferred way to do this? I never thought about using LVM for this before, but the last time I was in there I noticed some RAID options. This is for a Samba/LDAP/home directory server.



"Software RAID" is simply doing the RAID (striping, mirroring, parity, whatever) in the OS instead of on a dedicated card. Windows NT, from at least v3.50 (way back when), can do this, and Linux can do it as well.

If you're thinking of RAID 5, which is my preferred level, I'd avoid doing it in software and instead opt for a dedicated RAID card. Something like an LSI MegaRAID 150-6 SATA controller. If you do it in software, you'll eat up some CPU doing the parity calculations, so you definitely want to offload that. However, for just mirroring (say, RAID 1), you should be fine, because the CPU hit for mirroring is minimal.

One advantage to using a RAID card is that your array then appears as one big disk. You don't have to go messing around with mdadm or anything else.

Now, LVM is something else entirely and not really related to RAID. LVM is a way of partitioning disks that allows you to easily add or subtract space from what's called a "volume". A "volume" in an LVM setup appears to the OS as a disk partition. Windows NT calls this "volume groups". It works on individual disks as well as arrays. You can in fact take several disk arrays (say, three or four RAID 5's), throw all of them into one of these "volumes", and that entire volume appears as one partition-like entity to the OS. So, LVM is more of a SAN technology than a RAID technology.

Hope this helps,


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