On Thu, Sep 30, 2010 at 12:02 PM, chas williams - CONTRACTOR <c...@cmf.nrl.navy.mil> wrote: > On Thu, 30 Sep 2010 14:19:51 +0200 > Stephan Wiesand <stephan.wies...@desy.de> wrote: > >> Hi Jeff, >> >> On Sep 29, 2010, at 22:18 , Jeffrey Altman wrote: >> >> > RAID is not a replacement for ZFS. ZRAID-3 protects against single >> > bit disk corruption errors that RAID cannot. Only ZFS stores a >> > checksum of the data as part of each block and verifies it before >> > delivering the data to the application. If the checksum fails and >> > there are replicas, ZFS will read the data from another copy and >> > fixup the damaged version. That is what makes ZFS so special and so >> > valuable. If you have data that must be correct, you want ZFS. >> >> >> you're right, of course. This is a very desirable feature, and the >> main reason why I'd love to see ZFS become available on linux. >> >> I disagree on the "RAID cannot provide this" statement though. RAID-5 >> has the data to detect single bit corruption, and RAID-6 even has the >> data to correct it. Alas, verifying/correcting data upon read is not >> a common feature. I know of just one vendor (DDN) actually providing >> it. It's a mystery to me why the others don't. >> >> Anyway, the next best option if ZFS is not available is to run parity >> checks on all your arrays regularly. Things do get awkward when >> errors show up, but at least you know. Both Linux MD RAID and the >> better hardware solutions offer this. >> >> From my experience, disks don't do this at random and do not develop >> such a fault during their life span, but some broken disks do it >> frequently from the beginning. NB I only ever observed this problem >> with SATA drives. > > raid5 really isnt quite the same as what jeff is describing about zfs. > zfs apparently maintains multiple copies of the same block across > different devices. if you had a single bit error in one of the those > blocks (as determine by some checksum apparently stored with this > block), zfs will pick another block that is supposed to contain the > same data. > > raid5 only corrects single bit errors. it can detect multiple bit > errors. raid5 (to my knowledge) always verifies, even on reads and can > correct single bit errors. raid6 can correct two single bit
RAID-5 only provides a single parity bit. Unfortunately, this means that it can merely detect a single bit parity error; it cannot correct the error since there is insufficient information to prove which of the stripes is in error. RAID-6 is complicated because different implementations use different algorithms for the two orthogonal checksums. IIRC, all of them are able to detect two-bit errors, and some of them can correct a single-bit error. -Tom _______________________________________________ OpenAFS-info mailing list OpenAFS-info@openafs.org https://lists.openafs.org/mailman/listinfo/openafs-info