8.0-RELEASE: disk IO temporarily hangs up (ZFS or ATA related problem)

David P. Discher dpd at bitgravity.com
Wed Dec 23 20:37:52 UTC 2009

ZFS will write buffer ALOT.  Your settings of vfs.zfs.arc_min and vfs.zfs.arc_max will likely affect how bursty this is.  I've been running with Min, 32 GB arc, and see very bursty writes (of course, my systems have 64GB). 

Also, use 'zpool iostat 1' and not the system iostat. Check gstat as well.

also try using CAM_ATA (ahci.ko) instead of ad devices, so your can get the benefits of tag queuing.

There is also a possible undocumented issue with, I believe it was the WD green drives, where the vibration of the drive actually slowed it down.  This was demonstrated by setting a pound or two of silly putty on the top of the drive, and its performance (significantly) increased.  Experiment with putting some heavily, non-conductive weight on top of each drive - a book or something.  ensure that is its securely mounted as well. 

David P. Discher
dpd at bitgravity.com * AIM: bgDavidDPD
BITGRAVITY * http://www.bitgravity.com

On Dec 18, 2009, at 12:04 PM, Alexander Zagrebin wrote:

>>> The disks are used for mirrored ZFS pool.
>>> I have noticed that the system periodically locks up on 
>> disk operations.
>>> After approx. 10 min of very slow disk i/o (several KB/s) 
>> the speed of disk
>>> operations restores to normal.
>>> gstat has shown that the problem is in ad6.
>> Want you try this patch and report?:
>> http://people.freebsd.org/~attilio/lockmgr_lostwake.diff
>> It should apply to STABLE_8 and if not, just report and I will help.
> Thanks fo your attempt to help!
> I've successfully applied this patch to RELENG_8_0, recompiled
> kernel and required modules (krpc, opensolaris,zfs),
> but nothing has changed. The problem still exist.
> -- 
> Alexander Zagrebin
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