[Lustre-discuss] Hardware or software RAID?

[Peter Kjellstrom]

On Monday 24 November 2008, Jeff Layton wrote:
> Andreas Dilger wrote:
> > There are a number of large clusters (TACC Ranger in particular) that use
> > software RAID on JBODs, but the majority of systems use hardware RAID in
> > order to maximize performance (at an increased cost of course).
> >
> > These days I would tend to recommend using RAID-6 over RAID-5 just
> > because the large disks available take a long time to rebuild, and there
> > is a non-zero risk of a second disk failing during that time.
>
> What about using more, but smaller raid groups? For example,
> perhaps 4-5 drives in a RAID-5? That way if a disk fails, the
> rebuilds are faster since there is less data?

I'd pick raid6 not so much for the time-window/"drive fail" as for the read 
error rate. I've seen numbers for SATA drives at about once every 10TB or so. 
If so, then rebuilding a 10+1 raid5 is likely to (on average) see one sector 
read error (you're allowed 0 to manage a perfect rebuild). A 5+1 set would be 
50% likely to hit one and so on.

How your raid controller (or software) reacts to a failed sector read varies 
but behaviours include: continue as if nothing happened (you now have bad 
data on your raid-set), fail the offending drive (and then also the rebuild 
and the entire raid-set), ...

/Peter
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