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

[Chris Mason <chris.mason@oracle.com>]

On Fri, 2009-03-27 at 16:50 -0400, Chris Mason wrote:
> On Fri, 2009-03-27 at 16:24 -0400, Theodore Ts'o wrote:
> > The following patches have been posted as providing at least some
> > partial improvement to the ext3 latency problem that has been
> > discussed on the 2.6.29 mongo-LKML-thread-that-would-not-die.
> 
> 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.
> 
> Then it runs one fsync every 4 seconds and times how long they take.
> After the program has been running for 60 seconds, it tries to stop.
> 
> On my sata drive with barriers on, even btrfs and xfs saw some
> multi-second fsyncs, but ext3 came in at 414s for a single fsync.
> 
> Warning: don't run this on a laptop drive, you'll still be waiting for
> it next year.  This is probably full of little errors, I cut it together
> pretty quickly.
> 

My understanding of ext4 delalloc is that once blocks are allocated to
file, we go back to data=ordered.  

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.

-chris