Re: Why does btrfs benchmark so badly in this case?

[Josef Bacik <jbacik@fusionio.com>]

On Thu, Aug 08, 2013 at 09:13:04AM -0700, John Williams wrote:
> Phoronix periodically runs benchmarks on filesystems, and one thing I
> have noticed is that btrfs always does terribly on their fio "Intel
> IOMeter fileserver access pattern" benchmark:
> 
> http://www.phoronix.com/scan.php?page=article&item=linux_310_10fs&num=2
> 
> Here, btrfs is more than 6 times slower than ext4, and about 3 times
> slower than XFS.
> 
> Lest we attribute it to an unavoidable downside of COW filesystems and
> move on...no, we cannot do that, because ZFS does well here -- btrfs
> is about 6 times slower than ZFS!
> 
> Note that btrfs does quite well in the other Phoronix benchmarks. It
> is just the fio fileserver benchmark that btrfs has problems with.
> 
> What is going on here? Why is btrfs doing so poorly?

So the reason this workload sucks for btrfs is because we fall back on buffered
IO because fio does not do block size aligned writes for this workload.  If you
add

ba=4k

to the iometer fio file then we go the same speed as xfs and ext4.  Not a whole
lot we can do about this since unaligned writes means we have to read in pages
to cow the block properly, which is why we fall back to buffered.  Once we do
that we end up having a lot of page locking stuff that gets in the way and makes
us twice as slow.  Thanks,

Josef