On XFS write barriers

I’m very naive, when I have to trust software. I just can’t believe a filesystem may have a tunable that makes it 20x faster (or rather, doesn’t make it 20x slower). I expect it to work out of the box. So, I was pondering, why in my testing XFS on LVM flushes data ~20x faster than on a box where it talks directly to device. Though I have noticed some warnings before, people on #xfs pointed out that LVM doesn’t support write barriers.

So, as I had no idea what write barriers are, had to read up a bit on that. There is a very nice phrase in there regarding battery-backed write-behind caching:

Using write barriers in this instance is not warranted and will in fact lower performance. Therefore, it is recommended to turn off the barrier support and mount the filesystem with “nobarrier”.

No shit, 20x lower performance :) As usually, I was not the only one to spot that..

So, I just ran this:

mount -o remount,nobarrier /a

And InnoDB flushed pages at 80MB/s instead of 4MB/s.

Update (2009/03): 2.6.29 kernel will support write barriers for LVM too – so XFS performance degradation is very much expected at very very wide scope. Also ext4 uses write barriers by default too. This thing is getting huge.