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Re: Reducing memory requirements for high extent xfs files

To: David Chinner <dgc@xxxxxxx>
Subject: Re: Reducing memory requirements for high extent xfs files
From: Vlad Apostolov <vapo@xxxxxxx>
Date: Wed, 06 Jun 2007 12:00:28 +1000
Cc: Michael Nishimoto <miken@xxxxxxxxx>, Michael Nishimoto <miken@xxxxxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20070606013601.GR86004887@sgi.com>
References: <200705301649.l4UGnckA027406@oss.sgi.com> <20070530225516.GB85884050@sgi.com> <4665E276.9020406@agami.com> <20070606013601.GR86004887@sgi.com>
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David Chinner wrote:
On Tue, Jun 05, 2007 at 03:23:50PM -0700, Michael Nishimoto wrote:
David Chinner wrote:
On Wed, May 30, 2007 at 09:49:38AM -0700, Michael Nishimoto wrote:

Has anyone done any work or had thoughts on changes required
to reduce the total memory footprint of high extent xfs files?
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?
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.

Oh, that problem.

The issue is that allocation beyond EOF (the normal way we prevent
fragmentation in this case) gets truncated off on file close.

Even NFS request is processed by doing:


And so XFS truncates the allocation beyond EOF on close. Hence
the next write requires a new allocation and that results in
a non-contiguous file because the adjacent blocks have already
been used....


        - NFS server open file cache to avoid the close.
        - add detection to XFS to determine if the called is
          an NFS thread and don't truncate on close.
        - use preallocation.
        - preallocation on the file once will result in the
          XFS_DIFLAG_PREALLOC being set on the inode and it
          won't truncate on close.
        - append only flag will work in the same way as the
          prealloc flag w.r.t preventing truncation on close.
        - run xfs_fsr

Note - i don't think extent size hints alone will help as they
don't prevent EOF truncation on close.

I think extent hint should help in this situation. Here is an example
of writing 4 chars in a file with extent hint of 16Kb. The file ends
up with size of 4 and 8 basic blocks (512 bytes each) allocation in
one extent.

emu:/mnt/scratch1/temp # xfs_io -c "extsize 16384" -f foo
emu:/mnt/scratch1/temp # ls -al foo
-rw------- 1 root root 0 2007-06-06 12:33 foo
emu:/mnt/scratch1/temp # xfs_bmap -l -v foo
foo: no extents
emu:/mnt/scratch1/temp # echo "abc" > foo
emu:/mnt/scratch1/temp # ls -al foo
-rw------- 1 root root 4 2007-06-06 12:35 foo
emu:/mnt/scratch1/temp # xfs_bmap -l -v foo
  0: [0..7]:          326088..326095    0 (326088..326095)     8

Just a warning that the extent hint works at the moment only for
contiguous files. There are problems for sparse files (with holes)
and extent hint.


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