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Re: Reducing memory requirements for high extent xfs files

To: Michael Nishimoto <miken@xxxxxxxxx>
Subject: Re: Reducing memory requirements for high extent xfs files
From: Vlad Apostolov <vapo@xxxxxxx>
Date: Wed, 06 Jun 2007 09:11:05 +1000
Cc: David Chinner <dgc@xxxxxxx>, Michael Nishimoto <miken@xxxxxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <4665E276.9020406@agami.com>
References: <200705301649.l4UGnckA027406@oss.sgi.com> <20070530225516.GB85884050@sgi.com> <4665E276.9020406@agami.com>
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Michael Nishimoto wrote:

David Chinner wrote:
On Wed, May 30, 2007 at 09:49:38AM -0700, Michael Nishimoto wrote:
 > Hello,
 > Has anyone done any work or had thoughts on changes required
 > to reduce the total memory footprint of high extent xfs files?

We changed the way we do memory allocation to avoid needing
large contiguous chunks of memory a bit over a year ago;
that solved the main OOM problem we were getting reported
with highly fragmented files.

 > Obviously, it is important to reduce fragmentation as files
 > are generated and to regularly defrag files, but both of these
 > alternatives are not complete solutions.
 > To reduce memory consumption, xfs could bring in extents
 > from disk as needed (or just before needed) and could free
 > up mappings when certain extent ranges have not been recently
 > accessed.  A solution should become more aggressive about
 > reclaiming extent mapping memory as free memory becomes limited.

Yes, it could, but that's a pretty major overhaul of the extent
interface which currently assumes everywhere that the entire
extent tree is in core.

Can you describe the problem you are seeing that leads you to
ask this question? What's the problem you need to solve?


Dave Chinner
Principal Engineer
SGI Australian Software Group

I realize that this work won't be trivial which is why I asked if anyone has thought about all relevant issues.

When using NFS over XFS, slowly growing files (can be ascii log files)
tend to fragment quite a bit.  One system had several hundred files
which required more than one page to store the extents.  Quite a few
files had extent counts greater than 10k, and one file had 120k extents.
Besides the memory consumption, latency to return the first byte of the
file can get noticeable.


Hi Michael,

set extent hint size, which would reduce the file fragmentation in this scenario.
Please check xfcntl man page for more details.



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