Re: Apparently spurious ZFS CRC errors (was Re: ZFS data error without reasons)

On Wed, 25 Mar 2009, Alexander Leidinger wrote:

> Quoting Mark Powell <M.S.Powell_at_salford.ac.uk> (from Fri, 20 Mar 2009 
> 15:30:11 +0000 (GMT)):
>> Hmmm. Perhaps I'm not being fair on 8. Just had a look at my loader.conf 
>> for 7 and I can see that I used to run with every zfs*disable on. I've just 
>> rebooted 8 with:
>> 
>> vfs.zfs.cache_flush_disable: 1
>> vfs.zfs.mdcomp_disable: 1
>> vfs.zfs.prefetch_disable: 1
>> vfs.zfs.zil_disable: 1
>> hw.ata.wc: 1
>> 
>> The current fs which produced errors on every scrub now reports no errors.
>>  I now need to find which option fixed it.
>>  I suspect hw.ata.wc. Is this still a known issue?

Alex,
   Thanks for the input,

> I would expect that it is the combination of cache_flush_disable and 
> zil_disable with the wc enable. If you reenable the zil and the cache flush, 
> the wc should not cause the problems you see. You may want to have a look at 
> http://www.solarisinternals.com/wiki/index.php/ZFS_Evil_Tuning_Guide for a 
> more detailed description of what those options are doing (and why you should 
> not disable those features on normal disks). I also suggest to not disable 
> the meta-data compression, as it seems it only affects a small amount of 
> meta-data instead of all meta-data.

I initially ran 8 with default options i.e. write caching on, all zfs 
options left enabled. I got the errors. Only then did I try changing 
options, by turning off wc and disabling zfs options.
   It's a little curious that I should reenable the wc, and all zfs options 
except prefetch_disable. That takes me back to how I originally started, 
but with the solution to the problem therefore being disabling the 
prefetch.
   Can prefetch really cause these problems? And if so why?

> If you want to get more out of zfs, maybe vfs.zfs.vdev.max_pending could help 
> if you are using SATA (as I read the zfs tuning guide, it makes sense to have 
> a high value when you have command queueing, which we have with SCSI drives, 
> but not yet with SATA drives and probably not at all with PATA drives).

I'm running completely SATA with NCQ supporting drives. However, and 
possibly as you say, NCQ is not really/properly supported in FBSD?
   Many thanks for your time.

-- 
Mark Powell - UNIX System Administrator - The University of Salford
Information & Learning Services, Clifford Whitworth Building,
Salford University, Manchester, M5 4WT, UK.
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