linux-kernel - Re: io-scheduler tuning for better read/write ratio

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Message-ID: <x49iqinys2k.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 23 Jun 2009 09:53:39 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Ralf Gross <Ralf-Lists@...fgross.de>
Cc:	linux-kernel@...r.kernel.org, fengguang.wu@...el.com
Subject: Re: io-scheduler tuning for better read/write ratio

Ralf Gross <Ralf-Lists@...fgross.de> writes:

> Jeff Moyer schrieb:
>> Ralf Gross <rg@...-Softwaretechnik.com> writes:
>> 
>> > Jeff Moyer schrieb:
>> >> Jeff Moyer <jmoyer@...hat.com> writes:
>> >> 
>> >> > Ralf Gross <rg@...-softwaretechnik.com> writes:
>> >> >
>> >> >> Casey Dahlin schrieb:
>> >> >>> On 06/16/2009 02:40 PM, Ralf Gross wrote:
>> >> >>> > David Newall schrieb:
>> >> >>> >> Ralf Gross wrote:
>> >> >>> >>> write throughput is much higher than the read throughput (40 MB/s
>> >> >>> >>> read, 90 MB/s write).
>> >> >>> > 
>> >> >>> > Hm, but I get higher read throughput (160-200 MB/s) if I don't write
>> >> >>> > to the device at the same time.
>> >> >>> > 
>> >> >>> > Ralf
>> >> >>> 
>> >> >>> How specifically are you testing? It could depend a lot on the
>> >> >>> particular access patterns you're using to test.
>> >> >>
>> >> >> I did the basic tests with tiobench. The real test is a test backup
>> >> >> (bacula) with 2 jobs that create 2 30 GB spool files on that device.
>> >> >> The jobs partially write to the device in parallel. Depending which
>> >> >> spool file reaches the 30 GB first, one starts reading from that file
>> >> >> and writing to tape, while to other is still spooling.
>> >> >
>> >> > We are missing a lot of details, here.  I guess the first thing I'd try
>> >> > would be bumping up the max_readahead_kb parameter, since I'm guessing
>> >> > that your backup application isn't driving very deep queue depths.  If
>> >> > that doesn't work, then please provide exact invocations of tiobench
>> >> > that reprduce the problem or some blktrace output for your real test.
>> >> 
>> >> Any news, Ralf?
>> >
>> > sorry for the delay. atm there are large backups running and using the
>> > raid device for spooling. So I can't do any tests.
>> >
>> > Re. read ahead: I tested different settings from 8Kb to 65Kb, this
>> > didn't help.
>> >
>> > I'll do some more tests when the backups are done (3-4 more days).
>> 
>> The default is 128KB, I believe, so it's strange that you would test
>> smaller values.  ;)  I would try something along the lines of 1 or 2 MB.
>
> Err, yes this should have been MB not KB.
>
>
> $cat /sys/block/sdc/queue/read_ahead_kb 
> 16384
> $cat /sys/block/sdd/queue/read_ahead_kb 
> 16384
>
> I also tried different values for max_sectors_kb, nr_requests. But the
> trend that writes were much faster than reads while there was read and
> write load on the device didn't change.
>
> Changing the deadline parameter writes_starved, write_expire,
> read_expire, front_merges or fifo_batch didn't change this behavoir.

OK, bumping up readahead and changing the deadline parameters listed
should have give some better results, I would think.  Can you give the
invocation of tiobench you used so I can try to reproduce this?

Thanks!
Jeff
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