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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 15:52:11 +0200
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <51B84EB3.1070906@xxxxxxxxxx>
References: <loom.20130611T112155-970@xxxxxxxxxxxxxx> <51B72D3D.5010206@xxxxxxxxxx> <CAO9HMNGjdikgX+_434aGVJ2NAJ0hxDNLo+Vsa46GH3psXr4sKQ@xxxxxxxxxxxxxx> <1371025611.7096.10.camel@acme> <51B84EB3.1070906@xxxxxxxxxx>
On Wed, 2013-06-12 at 06:34 -0400, Ric Wheeler wrote:
> 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.

In progress. 

> > 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 have a small test app that can either write at full speed (100 MB/Sec
as it turns out on the system in question) or at the app rate of 25
MB/Sec. When at full speed, the cache becomes bigger faster. But the
result is the same. Memory eventually seems to be all taken by the
cache. It is freed when I umount the file system or "echo 1
> /proc/sys/vm/drop_caches"

Yours sincerely,

Roger Oberholtzer

RambÃll RST / Systems

Office: Int +46 10-615 60 20
Mobile: Int +46 70-815 1696

RambÃll Sverige AB
Krukmakargatan 21
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SE-104 62 Stockholm, Sweden

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