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Re: Questions about XFS

To: xfs@xxxxxxxxxxx
Subject: Re: Questions about XFS
From: Roger Oberholtzer <roger@xxxxxx>
Date: Wed, 12 Jun 2013 10:26:51 +0200
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <CAO9HMNGjdikgX+_434aGVJ2NAJ0hxDNLo+Vsa46GH3psXr4sKQ@xxxxxxxxxxxxxx>
References: <loom.20130611T112155-970@xxxxxxxxxxxxxx> <51B72D3D.5010206@xxxxxxxxxx> <CAO9HMNGjdikgX+_434aGVJ2NAJ0hxDNLo+Vsa46GH3psXr4sKQ@xxxxxxxxxxxxxx>
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.

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).

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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