12x performance drop on md/linux+sw raid1 due to barriers [xfs]

[Eric Sandeen]

Justin Piszcz wrote:
> Someone should write a document with XFS and barrier support, if I recall,
> in the past, they never worked right on raid1 or raid5 devices, but it
> appears now they they work on RAID1, which slows down performance ~12 times!!

What sort of document do you propose?  xfs will enable barriers on any
block device which will support them, and after:

deeb5912db12e8b7ccf3f4b1afaad60bc29abed9

[XFS] Disable queue flag test in barrier check.

xfs is able to determine, via a test IO, that md raid1 does pass
barriers through properly even though it doesn't set an ordered flag on
the queue.

> l1:~# /usr/bin/time tar xf linux-2.6.27.7.tar 
> 0.15user 1.54system 0:13.18elapsed 12%CPU (0avgtext+0avgdata 0maxresident)k
> 0inputs+0outputs (0major+325minor)pagefaults 0swaps
> l1:~#
> 
> l1:~# /usr/bin/time tar xf linux-2.6.27.7.tar
> 0.14user 1.66system 2:39.68elapsed 1%CPU (0avgtext+0avgdata 0maxresident)k
> 0inputs+0outputs (0major+324minor)pagefaults 0swaps
> l1:~#
> 
> Before:
> /dev/md2        /               xfs     defaults,noatime  0       1
> 
> After:
> /dev/md2        /               xfs     defaults,noatime,nobarrier,logbufs=8,logbsize=262144 0 1

Well, if you're investigating barriers can you do a test with just the
barrier option change; though I expect you'll still find it to have a
substantial impact.

> There is some mention of it here:
> http://oss.sgi.com/projects/xfs/faq.html#wcache_persistent
> 
> But basically I believe it should be noted in the kernel logs, FAQ or somewhere
> because just through the process of upgrading the kernel, not changing fstab
> or any other part of the system, performance can drop 12x just because the
> newer kernels implement barriers.

Perhaps:

printk(KERN_ALERT "XFS is now looking after your metadata very
carefully; if you prefer the old, fast, dangerous way, mount with -o
nobarrier\n");

:)

Really, this just gets xfs on md raid1 in line with how it behaves on
most other devices.

But I agree, some documentation/education is probably in order; if you
choose to disable write caches or you have faith in the battery backup
of your write cache, turning off barriers would be a good idea.  Justin,
it might be interesting to do some tests with:

barrier,   write cache enabled
nobarrier, write cache enabled
nobarrier, write cache disabled

a 12x hit does hurt though...  If you're really motivated, try the same
scenarios on ext3 and ext4 to see what the barrier hit is on those as well.

-Eric