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 when
> 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@xxxxxx>
> 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@xxxxxxxxxxxxx
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