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

[Jack Coats]

Raid6 sounds like a reasonable upgrade from Raid5

But as I have seen deomonstrated many times, no RAID level is better  
than good, tested, verified, backups to another medium in case of a  
'worst case scenario'.

There are many times I have had to recover data from various RAID  
arrays, that are not typically hardware issues. (One time I had a  
'hardware' issue, on disk died, a CE from the vendor -HP- came in to  
replace the drive at a clients site, and he pulled the wrong disk from  
the RAID5 array. ... We spent two days spinning tapes to recover. And  
he was invited to never darken the door of that clients office  
building again.)

Quoting Derek Atkins <warlord-DPNOqEs/LNQ at public.gmane.org>:

> Jarod Wilson <jarod-ajLrJawYSntWk0Htik3J/w at public.gmane.org> writes:
>
>> 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).
>
> Yeah, I read that much on wikipedia..
>
>> 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*).
>
> Fair enough..  For me I'm pondering this for the case of an 8-core
> vmware host server.
>
>>> 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...
>
> IIRC:  http://www.miracleas.com/BAARF/RAID5_versus_RAID10.txt
>
>> Jarod Wilson
>> jarod-ajLrJawYSntWk0Htik3J/w at public.gmane.org
>
> -derek
>
> --
>        Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
>        Member, MIT Student Information Processing Board  (SIPB)
>        URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
>        warlord-DPNOqEs/LNQ at public.gmane.org                        PGP key available
>
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