[K12OSN] Linux "Software RAID"

[Rob Owens]

On Thu, Aug 07, 2008 at 03:20:21PM -0400, Terrell Prude' Jr. wrote:
> 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.
> 
I hear lots of people talk about the CPU hit of software RAID.  But how much hit is there really?  Suppose for argument's sake I can get a hardware RAID card for $100.  If I instead used software RAID and spent my $100 on a better CPU, wouldn't I be ahead of the game?

-Rob