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.
Best regards,
Stephan
--
Stephan Wiesand
DESY -DV-
Platanenenallee 6
15738 Zeuthen, Germany
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