On 06/14/2011 12:06 PM, Eric Sandeen wrote: > On 6/14/11 10:41 AM, Allison Henderson wrote: >> 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 > > so the file should go through these steps: > (H=hole, P=prealloc, D=data) > > 0k 20k > | H | H | H | H | H | (truncate) > | P | P | P | P | P | (alloc_cmd) > | P | P | D | P | P | (pwrite) > <fsync> (fsync) > | P | H | H | H | P | (punch) > >> 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. > > so it's like this? > > 0: [0..7]: data > 1: [8..31]: hole > 2: [32..39]: data > >> 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. > > Yes, it does do that IIRC. > > I probably need to look closer, but any test which expects exact > layouts from a filesystem after a series of operations is probably > expecting too much... > > From a data integrity perspective, written zeros is as good as a hole is > as good as preallocated space, so I suppose those should all be acceptable, > though I guess "punch" should result in holes exactly as requested. > >> 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! > > Josef, what do you think? It's your test originally. :) > Yes, a test that was really only meant to test the block based fiemap since they all act in a dumb and easy to verify way. I think if we want to keep this test we should probably have it just recognize these little optimizations so it doesn't freak out. Thanks, Josef -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
