ZFS scrub 'repaired' pool with no chksum or read errors?
Jeremy Chadwick
freebsd at jdc.parodius.com
Fri Jun 10 09:33:20 UTC 2011
On Fri, Jun 10, 2011 at 09:43:14AM +0100, Karl Pielorz wrote:
> I'm running FreeBSD-8.2R amd64 w/4Gb of ECC RAM on a machine used
> for 'offsite' backups (that are copied to it using zfs
> send/receive).
>
> I scrub this machine every now and again (about once a month) -
> recently this resulted in the following output after the scrub
> completed:
>
> "
> # zpool status
> pool: vol
> state: ONLINE
> scrub: scrub completed after 2h49m with 0 errors on Thu Jun 9
> 17:09:31 2011
> config:
>
> NAME STATE READ WRITE CKSUM
> vol ONLINE 0 0 0
> raidz1 ONLINE 0 0 0
> ada0 ONLINE 0 0 0 256K repaired
> ada1 ONLINE 0 0 0
> ada2 ONLINE 0 0 0
>
> errors: No known data errors
> "
>
> Should I be worried there was 256k of 'repairs' done, even though
> there were no checksum errors, or read errors detected?
>
> The console logged no errors - and nothing shows in syslog.
>
> The machine is always cleanly shut down - and the drives all appear
> fine from a SMART point of view - I'm just a bit concerned as to
> where the repairs came from - as ZFS doesn't seem to know (or be
> able to tell me) either :)
ZFS experts please correct me, but my experience with this has shown me
that the scrub itself found actual issues while analysing all data on
the entire pool -- more specifically, I believe READ/WRITE/CKSUM are
counters used for when errors are encountered during normal (read:
non-scrub) operations. It's been a while since I've seen this happen,
but have seen it on our Solaris 10 machines at my workplace. I've never
been sure what it means; possibly signs of "bit rot"?
If you're worried about your disk (ada0), please provide output from
"smartctl -a /dev/ada0" and I'll be more than happy to review the output
and provide you with any insights. I do believe you when you say it
looks fine, but every model of disk is different in some regard.
--
| Jeremy Chadwick jdc at parodius.com |
| Parodius Networking http://www.parodius.com/ |
| UNIX Systems Administrator Mountain View, CA, US |
| Making life hard for others since 1977. PGP 4BD6C0CB |
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