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SubjectRe: ext3 IO latency measurements (was: Linux 2.6.29)
On Thu 26-03-09 15:57:25, Andrew Morton wrote:
> On Thu, 26 Mar 2009 15:39:53 -0700 (PDT) Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> > On Thu, 26 Mar 2009, Jan Kara wrote:
> > >
> > > Reads are measurably better with the patch - the test with cat you
> > > describe below took ~0.5s per file without the patch and always less than
> > > 0.02s with the patch. So it seems to help something.
> >
> > That would seem to be a _huge_ improvement.
>
> It's strange that we still don't have an ext3_writepages(). Open a
> transaction, do a large pile of writes, close the transaction again.
> We don't even have a data=writeback writepages() implementation, which
> should be fairly simple.
Doable but not fairly simple ;) Firstly you have to restart a transaction
when you've used up all the credits you originally started with (easy),
secondly ext3 uses lock order PageLock -> "transaction start" which is
unusable for the scheme you suggest. So we'd have to revert that - which
needs larger audit of our locking scheme and that's probably the reason
why noone has done it yet.

> Bizarre.
>
> Mingming had a shot at it a few years ago and I think Badari did as
> well, but I guess it didn't work out.
>
> Falling back to generic_writepages() on our main local fs is a bit lame.

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR


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