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List:       ceph-devel
Subject:    Re: async+direct IO in osd journal write
From:       Simon Tian <aixt2006 () gmail ! com>
Date:       2011-08-22 3:08:56
Message-ID: CAFGK4NJOr4XLBfJhpHrco6y4aMcXdvMZ+Ug-XRGqP1AbNP2X2w () mail ! gmail ! com
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Hi,

For rand small write, libaio or posix direct aio would be very nice
for the IOPS, I think this kind of journal should be the same.

For data read and write, libaio will perform very nice for rand small
read and write. So If user create a rbd image for a KVM image, IOPS
will be much better if OSD handle IO with libaio. Bandwidth will be a
little better too.

Thx
Simon

2011/8/20 Sage Weil <sage@newdream.net>:
> On Sat, 20 Aug 2011, huang jun wrote:
>> hi,all
>> Recently,we have a test to compare  async i/o and sync i/o in OSD.
>> The osd jounral was written in sync + direct io mode now, that heavily
>> infuluenced the write performance.
>> we use linux aio API. test file size is 40MB,and disk isn't busy.
>> here is the test result:
>>           type                  ext3
>> sync write+direct io      4.441s
>>
>> async write +direct io   0.462s
>>
>> we are glad to see that, so we want to affirm that :
>> 1) can we use async+direct io to achieve better write performance if
>> we used osd journal?
>
> Do you mean changing the writes in os/FileJournal.cc to be async?  As long
> as the journal does not acknowledge a write until it is stable on disk,
> that is fine.  Async IO would be a nice improvement here; I think
> currently it's a single sync O_DIRECT io in flight at a time.
>
> The test is to write a 40MB file via which interface?  Via a kernel client
> mount?
>
> Thanks-
> sage
>
>
>> 2) does this  bring some problems?
>>
>> thank you in advance!
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