Re: Phoronix article slaming BTRFS

Chris Mason
2009-06-23T14:41:23Z

On Tue, Jun 23, 2009 at 02:51:41AM +0000, Mike Ramsey wrote:
> I ran across this article "Testing Out The SSD Mode In Btrfs".
> http://www.phoronix.com/scan.php?page=article&item=btrfs_ssd_mode&num=1
>
> At first I was disappointed. It gave a very disappointing set of benchmarks.
> However, a close reading revealed this:
>
> "With the OCZ Vertex SATA 2.0 SSD, which we used for this testing today, had its
> write caching always enabled. When attempting to disable the write cache through
> hdparm it would remain enabled regardless and when using sdparm it would report
> change_mode_page: failed setting page: Caching (SBC)."
>
> This invalidates the benchmark! Disabling the write cache would yield a 2X
> improvement.

Hmmm, I'm not sure I follow. I'm guessing the write cache is critical
on the vertex drives because they are using it to queue up writes into
large enough units to fill an entire erasure block at once. If they
took the time to put 64MB of the stuff in there, it probably does
something good ;)

Jens Axboe tried to reproduce the phoronix results on his ocz drive, and
generally found that each run was slower than the last regardless of
which mount options were used. This isn't entirely surprising, but it
did make it very difficult to nail down good or bad performance.

-chris