RAID6? (was Re: Anyone Actually Using Virtual Linux Servers?)

[Jarod Wilson]

On Tuesday 11 September 2007 10:48:05 am Derek Atkins wrote:
> Jarod Wilson <jarod-ajLrJawYSntWk0Htik3J/w at public.gmane.org> writes:
> > My personal use cases are my web and mail server and my myth backend.
> > RAID6 for me in both (well, the video store on my myth backend is
> > actually raid5 w/a hot spare, since my 3ware card is a slightly older
> > one that doesn't do raid6).
>
> I've heard of RAID0, RAID1, RAID10, and RAID5..  But what is RAID6 and
> how does it compare to the other RAIDs?  According to wikipedia it's
> like RAID5 but with dual-parity.

Yes, it requires at least 4 disks instead of 3 disks like RAID5, and your 
capacity is number of drives minus two. With RAID5, in the 3-disk case, you 
have two data stripes and one parity stripe for every write. With RAID6 in 
the 4-disk case, you have two data stripes and two parity stripes for every 
write. With that in place, RAID6 can survive a double-disk failure of any two 
disks in the array, whereas with RAID10, certain double-disk failures can 
still be fatal (and of course in RAID5, they're always fatal).

Personally, I moved from RAID5 to RAID6 after getting bit by a second disk 
going haywire while a RAID5 array was being rebuilt after replacing a disk 
that had failed. Went RAID6 over RAID10 primarily since it should survive 
failing any two disks, and in cursory testing, the throughput was still more 
than adequate (some have complained about the computational intensity of 
RAID6, but with a modern multi-core cpu, *shrug*).

> ISTR a problem in that with RAID5 if 
> you had a partial disk failure (i.e. it just returned bogus data) that
> you could corrupt your data because Linux S/W RAID didn't do checksum
> verification on reads -- so it was "safer" to use RAID10.  Is this
> "fixed" in raid6?

Hrm. I wasn't aware of any such issue, so I haven't a clue if its fixed or 
not...

-- 
Jarod Wilson
jarod-ajLrJawYSntWk0Htik3J/w at public.gmane.org

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