On Sat, 2008-12-06 at 09:36 -0600, Eric Sandeen wrote: > 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. I have tested with ext3/xfs, and barriers have considerably more impact on xfs than ext3. this is ~4 months old test, I do not have any precise data anymore. > > -Eric > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
