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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