fixing slow sync(2)

[Tomas Carnecky <tom@dbservice.com>]

The issue with sync(2) being slow has been known for a while now (I can 
find discussions as early as July 2010). But nobody seems to be willing 
to fix it or even figure out out what the actual problem is. I use btrfs 
on two hosts, and the sync(2) syscall is consistently slow. By slow I 
mean 20 seconds on one and 45(!) seconds on the other. Every single 
time, no exceptions.

Is there anything I can do to help track down the cause? I can test 
patches, trace the kernel or use whatever tool is most helpful.

tom