[K12OSN] Linux "Software RAID"

[Terrell Prude' Jr.]

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.
>
> Thanks,
>
> ck

"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,

--TP