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Re: xfs speculative preallocation -- fragmentation issue with sparse fil

To: Brian Foster <bfoster@xxxxxxxxxx>
Subject: Re: xfs speculative preallocation -- fragmentation issue with sparse file handling?
From: Mark Tinguely <tinguely@xxxxxxx>
Date: Mon, 18 Feb 2013 15:24:49 -0600
Cc: xfs@xxxxxxxxxxx, Dave Chinner <dchinner@xxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <51229835.5090707@xxxxxxxxxx>
References: <51229835.5090707@xxxxxxxxxx>
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On 02/18/13 15:08, Brian Foster wrote:
Hi guys,

I was running a sanity check of my quota throttling stuff rebased
against the updated speculative prealloc algorithm:

a1e16c26 xfs: limit speculative prealloc size on sparse files

... and ran into an interesting behavior on my baseline test (quota
disabled).

The test I'm running is a concurrent write of 32 files (10GB each) via
iozone (I'm not testing performance, just using it as a concurrent writer):

iozone -w -c -e -i 0 -+n -r 4k -s 10g -t 32 -F /mnt/data/file{0..31}

... what I noticed is that from monitoring du during the test,
speculative preallocation seemed to be ineffective. From further
tracing, I observed that imap[0].br_blockcount in
xfs_iomap_eof_prealloc_initial_size() was fairly consistently maxed out
at around 32768 blocks (128MB).

Without the aforementioned commit, preallocation occurs as expected and
the files result in 7-9 extents after the test. With the commit, I'm in
the 70s to 80s range of number of extents with a max extent size of
128MB. A couple examples of xfs_bmap output are appended to this
message. It seems like initial fragmentation might be throwing the
algorithm out of whack..?

Brian

... the patched version increases in doubles

+       if (imap[0].br_startblock == HOLESTARTBLOCK)
+               return 0;

    vvvvvv
+       if (imap[0].br_blockcount <= (MAXEXTLEN >> 1))
+               return imap[0].br_blockcount;
    ^^^^^^

+       return XFS_B_TO_FSB(mp, XFS_ISIZE(ip));
+}

have you experimented without the middle if statement.
If I remember correctly when I reviewed the code, that should be moving
code closer to the original code; namely use the file size as the
preallocation value.

--Mark.

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