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

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Message-ID: <x49d494c4u0.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 16 Jun 2009 16:16:07 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Ralf Gross <rg@...-softwaretechnik.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: io-scheduler tuning for better read/write ratio

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.

Cheers,
Jeff
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