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 -- 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
