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Re: Questions about XFS

To: xfs@xxxxxxxxxxx
Subject: Re: Questions about XFS
From: Ric Wheeler <rwheeler@xxxxxxxxxx>
Date: Wed, 12 Jun 2013 06:34:27 -0400
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In-reply-to: <1371025611.7096.10.camel@acme>
References: <loom.20130611T112155-970@xxxxxxxxxxxxxx> <51B72D3D.5010206@xxxxxxxxxx> <CAO9HMNGjdikgX+_434aGVJ2NAJ0hxDNLo+Vsa46GH3psXr4sKQ@xxxxxxxxxxxxxx> <1371025611.7096.10.camel@acme>
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On 06/12/2013 04:26 AM, Roger Oberholtzer wrote:
On Tue, 2013-06-11 at 11:12 -0500, Steve Bergman wrote:
Are you saying that with XFS there is no periodic
flushing mechanism at all? And that unless there's an
fsync/fdatasync/sync or the memory needs to be reclaimed, that it can
sit in the page cache forever?
I read the later responses to this and they seemed to say that the data
in the page cache should be written to the disk periodically. I am not
meaning to hijack the thread. I just have a question directly related to
this point.

You most likely need to adjust some of the vm tunings to cause the vm to kick out pages more evenly. Not sure what the opensuse crowd would suggest tweaking.

I have an application that is streaming data to an XFS disk at a
sustained 25 MB/sec. This is well below what the hardware supports. The
application does fopen/fwrite/fclose (no active flushing or syncing).

Sounds like this is more likely to be an application issue than a file system one. Can you push the IO write speed up with a simple "dd" test to a file?


I see that as my application writes data (the only process writing the
only open file on the disk), the system cache grows and grows. Here is
the unusual part: periodically, writes take some number of seconds to
complete, rather than the typical <50 msecs). The increased time seems
to correspond to the increasing size of the page cache.

If I do:

echo 1 > /proc/sys/vm/drop_caches

while the application is runnung, then the writes do not occasionally
take longer. Until the cache grows again, and I do the echo again.

I am sure I must be misinterpreting what I see.

(on openSUSE 12.1. kernel 3.1.0)

Roger Oberholtzer

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