Re: ext3 IO latency measurements (was: Linux 2.6.29)

[Andrew Morton]

On Fri, 27 Mar 2009 15:10:56 -0700 (PDT) Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Fri, 27 Mar 2009, Jan Kara wrote:
> >
> >   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.
> 
> It's also not clear that ext3 can really do much better than the regular 
> generic_writepages() logic. I mean, seriously, what's there to improve on? 

- opening a single transaction for many pages in the cases when a
  transaction _is_ needed.

- single large BIO versus zillions of single-page BIOs.

Relatively minor benefits, but it's a bit odd that we never got around
to doing it.

It just got quite a bit harder to do, so I expect we won't be doing it.

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