[PATCH v2] xfs: Avoid pathological backwards allocation

[Mark Tinguely]

On 05/20/13 08:56, Jan Kara wrote:
> On Thu 11-04-13 22:09:56, Jan Kara wrote:
>> Writing a large file using direct IO in 16 MB chunks sometimes results
>> in a pathological allocation pattern where 16 MB chunks of large free
>> extent are allocated to a file in a reversed order. So extents of a file
>> look for example as:
>>
>>   ext logical physical expected length flags
>>     0        0        13          4550656
>>     1  4550656 188136807   4550668 12562432
>>     2 17113088 200699240 200699238 622592
>>     3 17735680 182046055 201321831   4096
>>     4 17739776 182041959 182050150   4096
>>     5 17743872 182037863 182046054   4096
>>     6 17747968 182033767 182041958   4096
>>     7 17752064 182029671 182037862   4096
>> ...
>> 6757 45400064 154381644 154389835   4096
>> 6758 45404160 154377548 154385739   4096
>> 6759 45408256 252951571 154381643  73728 eof
>>
>> This happens because XFS_ALLOCTYPE_THIS_BNO allocation fails (the last
>> extent in the file cannot be further extended) so we fall back to
>> XFS_ALLOCTYPE_NEAR_BNO allocation which picks end of a large free
>> extent as the best place to continue the file. Since the chunk at the
>> end of the free extent again cannot be further extended, this behavior
>> repeats until the whole free extent is consumed in a reversed order.
>>
>> For data allocations this backward allocation isn't beneficial so make
>> xfs_alloc_compute_diff() pick start of a free extent instead of its end
>> for them. That avoids the backward allocation pattern.
>>
>> See thread at http://oss.sgi.com/archives/xfs/2013-03/msg00144.html for
>> more details about the reproduction case and why this solution was
>> chosen.
>>
>> Based on idea by Dave Chinner<dchinner at redhat.com>.
>>
>> CC: Dave Chinner<dchinner at redhat.com>
>> Reviewed-by: Dave Chinner<dchinner at redhat.com>
>> Signed-off-by: Jan Kara<jack at suse.cz>
>> ---
>>   fs/xfs/xfs_alloc.c |   24 ++++++++++++++++++------
>>   1 files changed, 18 insertions(+), 6 deletions(-)
>>
>> v2: Updated comment and commit description.
>    Could anybody pull this patch into XFS tree? I don't see it there...
>
> 								Honza

Sorry, a miscommunication on my part that this belonged in the dev tree 
but not in the for Linus pull for Linux 3.10.

--Mark.