[OpenIndiana-discuss] Mitigating the performance impact of scrub
richard.elling at richardelling.com
Thu Sep 27 20:46:29 UTC 2012
On Sep 27, 2012, at 11:51 AM, Reginald Beardsley <pulaskite at yahoo.com> wrote:
> Where would I find information about the kernel level tunable parameters? Are there specific parameters you have in mind?
The most important tend to be:
zfs:zfs_scrub_delay = 4 number of ticks to delay scrub (10ms/tick by default)
zfs:zfs_scan_idle = 50 idle window size (ticks)
to set on a running system, the kernel parameters are:
The algorithm intends to check to see if there are important I/Os within
zfs_scan_idle time, and if so delay scrub I/O by zfs_scrub_delay.
> What I'm seeing is very slow loads of OpenOffice. It's certainly not quick under the best of circumstances, but this was particularly slow. I just needed to read a bunch of 5-6 MB .xls files and export them as .csv files.
If you are using HDDs, then zfs_vdev_max_pending might be too high. To understand
> --- On Thu, 9/27/12, Richard Elling <richard.elling at richardelling.com> wrote:
>> From: Richard Elling <richard.elling at richardelling.com>
>> Subject: Re: [OpenIndiana-discuss] Mitigating the performance impact of scrub
>> To: "Discussion list for OpenIndiana" <openindiana-discuss at openindiana.org>
>> Date: Thursday, September 27, 2012, 1:15 PM
>> On Sep 27, 2012, at 8:44 AM, Reginald
>> Beardsley <pulaskite at yahoo.com>
>>> The only thing google turned up was "stop the scrub if
>> it impacts performance too badly" which is not really all
>> that helpful. Or ways to speed up scrubs & resilvers.
>> On modern ZFS implementations, scrub I/O is throttled to
>> avoid impacting application I/O
>> performance. There are some kernel-level tunable parameters
>> that can be adjusted, but
>> AFAIK, nobody has done any characterization studies.
>>> In my case, I'd like to be able to run a scrub and have
>> all the performance hits taken by the scrub process if the
>> system has other loads. I care that the scrub runs,
>> but not how fast.
>>> From observed behavior, it appears that the scrub is
>> consuming too large a share of DRAM (12 GB in this
>> case). Is that correct and if so, is there a way to
>> limit the proportion of memory used by the scrub process?
>> Data is cached in the ARC, but scrub data is placed in the
>> MRU/LRU side of the ARC
>> and shouldn't impact the MFU side. In most cases, the memory
>> usage during a scrub
>> is not a problem. Have you seen a different behaviour?
>> -- richard
>>> I'd like to be able to schedule scrubs regularly, but
>> the present behavior would require predicting when I didn't
>> want to use the system for work.
>> illumos Day & ZFS Day, Oct 1-2, 2012 San Fransisco
>> Richard.Elling at RichardElling.com
>> OpenIndiana-discuss mailing list
>> OpenIndiana-discuss at openindiana.org
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