From the discussion at Mike Shaver's blog on this -
http://shaver.off.net/diary/2008/05/25/fsyncers-and-curve... - it seems that this will be
fixed through a short-term FF3 patch and longer-term work on ext4 for fsync to be file-local.
For some reason O_SYNC is slower than fsync, but I still think it would be better to use O_SYNC
as it's guaranteed to be file-local and the performance should still be OK.