Re: [PATCH 0/3] Ext3 latency improvement patches

Aneesh Kumar K.V
2009-03-30T11:23:50Z

On Fri, Mar 27, 2009 at 05:30:52PM -0400, Theodore Tso wrote:
> On Fri, Mar 27, 2009 at 05:03:38PM -0400, Chris Mason wrote:
> > > Ric had asked me about a test program that would show the worst case
> > > ext3 behavior. So I've modified your ext3 program a little. It now
> > > creates a 8G file and forks off another proc to do random IO to that
> > > file.
> > >
> >
> > My understanding of ext4 delalloc is that once blocks are allocated to
> > file, we go back to data=ordered.
>
> Yes, that's correct.
>
> > Ext4 is going pretty slowly for this fsync test (slower than ext3), it
> > looks like we're going for a very long time in
> > jbd2_journal_commit_transaction -> write_cache_pages.
>
> One of the things that we can do to optimize this case for ext4 (and
> ext3) is that if block has already been written out to disk once, we
> don't have to flush it to disk a second time. So if we add a new
> buffer_head flag which can distinguish between blocks that have been
> newly allocated (and not yet been flushed to disk) versus blocks that
> have already been flushed to disk at least once, we wouldn't need to
> force I/O for blocks in the latter case.

write_cache_pages should only look at pages which are marked dirty right
?. So why are we writing these pages again and again ?

-aneesh