[OpenIndiana-discuss] Mitigating the performance impact of scrub

Daniel Kjar dkjar at elmira.edu
Thu Sep 27 19:20:45 UTC 2012

I see the same on my file server when I scrub the home pool.  I can't do 
it when users are on as it slows down quite a bit.  That is just the 
file server, the users are running off a devoted 8 processor machine 
with 32gb of ram, the file server just serves them their home folders 
but the I/O must be considerably slowed for requests from the users.  
Openoffice and firefox and rhythmbox, all of them get hit hard during a 
scrub so I assume it's anything that is asking the fileserver for disk 
access (nfs).

On 09/27/12 02:51 PM, Reginald Beardsley wrote:
> Where would I find information about the kernel level tunable parameters?  Are there specific parameters you have in mind?
> 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.
> Reg
> --- 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>
>> wrote:
>>> 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.
>>> Thanks,
>>> Reg
>> --
>> illumos Day & ZFS Day, Oct 1-2, 2012 San Fransisco
>> www.zfsday.com
>> Richard.Elling at RichardElling.com
>> +1-760-896-4422
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Dr. Daniel Kjar
Assistant Professor of Biology
Division of Mathematics and Natural Sciences
Elmira College
1 Park Place
Elmira, NY 14901

"...humans send their young men to war; ants send their old ladies"
	-E. O. Wilson

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