HUGE XFS regression in 2.6.32 upto 2.6.38

[Raz]

you are correct. man page says "..untill data has been physically
written to the underlying storage".
missed that one.

thank you dave

On Tue, Apr 12, 2011 at 12:52 PM, Dave Chinner <david at fromorbit.com> wrote:
> On Tue, Apr 12, 2011 at 10:58:53AM +0300, Raz wrote:
>> Christoph Hello
>> I am testing 2.6.38 with AIM benchmark.
>> I compared 2.6.38 to 2.6.27 and I noticed that 2.6.27 is much better
>> than 2.6.38 hwhen
>> doing sync random writes test over an xfs regular file over native
>> Linux partition on top common sata disk.
>> I git bisected the problem and I reached this SHA1:
>> commit 13e6d5cdde0e785aa943810f08b801cadd0935df
>> Author: Christoph Hellwig <hch at lst.de>
>> Date:   Mon Aug 31 21:00:31 2009 -0300
>>
>>    xfs: merge fsync and O_SYNC handling
>>
>>    The guarantees for O_SYNC are exactly the same as the ones we need to
>>    make for an fsync call (and given that Linux O_SYNC is O_DSYNC the
>>    equivalent is fdadatasync, but we treat both the same in XFS), except
>>    with a range data writeout.  Jan Kara has started unifying these two
>>    path for filesystems using the generic helpers, and I've started to
>>    look at XFS.
>> ...
>>
>>
>> The bellow two tests presents the how different performance is before and patch:
>> #test 16) bisect 11
>> ------------------------------------------------------------------------------------------------------------
>>  Test        Test        Elapsed  Iteration    Iteration          Operation
>> Number       Name      Time (sec)   Count   Rate (loops/sec)    Rate (ops/sec)
>> ------------------------------------------------------------------------------------------------------------
>>     1 sync_disk_rw        30.71         19    0.61869         1583.85
>> Sync Random Disk Writes (K)/second
>> ------------------------------------------------------------------------------------------------------------
>
> That's clearly showing that your sync writes are not hitting the
> disk. IOWs, the sync writes are not synchronous at all. There is
> no way a single SATA drive can do >1500 writes to stable storage
> per second.
>
> IOWs, before this fix, sync writes were broken on your hardware.
>
>> #test 17 ) bisect 12
>> ------------------------------------------------------------------------------------------------------------
>>     1 sync_disk_rw        69.05          1    0.01448           37.07
>> Sync Random Disk Writes (K)/second
>> ------------------------------------------------------------------------------------------------------------
>
> And that's pretty tpyical for a SATA drive where sync writes are
> actually hitting the platter correctly.
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david at fromorbit.com
>