Re: Best stripe-size in array for ZFS mail storage?
can you guess? <billtodd <at> metrocast.net>
2007-12-01 11:31:56 GMT
> Any reason why you are using a mirror of raid-5
Some people aren't willing to run the risk of a double failure - especially when recovery from a single
failure may take a long time. E.g., if you've created a disaster-tolerant configuration that separates
your two arrays and a fire completely destroys one of them, you'd really like to be able to run the survivor
without worrying too much until you can replace its twin (hence each must be robust in its own right).
The above situation is probably one reason why 'RAID-6' and similar approaches (like 'RAID-Z2') haven't
generated more interest: if continuous on-line access to your data is sufficiently critical to consider
them, then it's also probably sufficiently critical to require such a disaster-tolerant approach
(which dual-parity RAIDs can't address).
It would still be nice to be able to recover from a bad sector on the single surviving site, of course, but you
don't necessarily need full-blown RAID-6 for that: you can quite probably get by with using large blocks
and appending a private parity sector to them (maybe two private sectors just to accommodate a situation
where a defect hits both the last sector in the block and the parity sector that immediately follows it; it
would also be nice to know that the block size is significantly smaller than a disk track size, for similar
reasons). This would, however, tend to require file-system involvement such that all data was organized
into such large blocks: otherwise, all writes for smaller blocks would turn into read/modify/writes.
Panasas (I always tend to put an extra 's' into that name, and to judge from Google so do a hell of a lot of other
people: is it because of the resemblance to 'parnassas'?) has been crowing about something that it calls
'tiered parity' recently, and it may be something like the above.
> How about running a ZFS mirror over RAID-0 luns? Then
> again, the
> downside is that you need intervention to fix a LUN