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xfstests 252 failure

To: xfs-oss <xfs@xxxxxxxxxxx>, Ext4 Developers List <linux-ext4@xxxxxxxxxxxxxxx>, linux-fsdevel <linux-fsdevel@xxxxxxxxxxxxxxx>
Subject: xfstests 252 failure
From: Allison Henderson <achender@xxxxxxxxxxxxxxxxxx>
Date: Tue, 14 Jun 2011 08:41:27 -0700
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20110414 Thunderbird/3.1.10
Hi all,

I just wanted to get some ideas moving on this question before too much time goes by. Ext4 is currently failing xfstest 252, test number 12. Currently test 12 is:

        $XFS_IO_PROG $xfs_io_opt -f -c "truncate 20k" \
                -c "$alloc_cmd 0 20k" \       
                -c "pwrite 8k 4k" -c "fsync" \              
                -c "$zero_cmd 4k 12k" \
                -c "$map_cmd -v" $testfile | $filter_cmd
        [ $? -ne 0 ]&&  die_now

and the output is:

        12. unwritten -> data -> unwritten
0: [0..7]: unwritten
1: [8..31]: hole
2: [32..39]: unwritten

Ext4 gets data extents here instead of unwritten extents. I did some investigating and it looks like the fsync command causes the extents to be written out before the punch hole operation starts. It looks like what happens is that when an unwritten extent gets written to, it doesnt always split the extent. If the extent is small enough, then it just zeros out the portions that are not written to, and the whole extent becomes a written extent. Im not sure if that is incorrect or if we need to change the test to not compare the extent types.

It looks to me that the code in ext4 that does this is supposed to be an optimization to help reduce fragmentation. We could change the filters to print just "extent" instead of "unwritten" or "data", but I realize that probably makes the test a lot less effective for xfs. If anyone can think of some more elegant fixes, please let me know. Thx!

Allison Henderson

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