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SubjectRe: Linux 2.6.29

On Wed, 25 Mar 2009, Theodore Tso wrote:
> I still think the fsync() problem is the much bigger deal, and solving
> the contention problem isn't going to solve the fsync() latency problem
> with ext3 data=ordered mode.

The fsync() problem is really annoying, but what is doubly annoying is
that sometimes one process doing fsync() (or sync) seems to cause other
processes to hickup too.

Now, I personally solved that problem by moving to (good) SSD's on my
desktop, and I think that's indeed the long-term solution. But it would be
good to try to figure out a solution in the short term for people who
don't have new hardware thrown at them from random companies too.

I suspect it's a combination of filesystem transaction locking, together
with the VM wanting to write out some unrelated blocks or inodes due to
the system just being close to the dirty limits. Which is why the
system-wide hickups then happen especially when writing big files.

The VM _tries_ to do writes in the background, but if the writepage() path
hits a filesystem-level blocking lock, that background write suddenly
becomes largely synchronous.

I suspect there is also some possibility of confusion with inter-file
(false) metadata dependencies. If a filesystem were to think that the file
size is metadata that should be journaled (in a single journal), and the
journaling code then decides that it needs to do those meta-data updates
in the correct order (ie the big file write _before_ the file write that
wants to be fsync'ed), then the fsync() will be delayed by a totally
irrelevant large file having to have its data written out (due to
data=ordered or whatever).

I'd like to think that no filesystem designer would ever be that silly,
but I'm too scared to try to actually go and check. Because I could well
imagine that somebody really thought that "size" is metadata.


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