linux-ext4 - Re: [PATCH 0/3] Ext3 latency improvement patches

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Date:	Fri, 27 Mar 2009 22:19:15 +0100
From:	Jan Kara <jack@...e.cz>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Theodore Ts'o <tytso@....edu>, Ric Wheeler <rwheeler@...hat.com>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	jack@...e.cz
Subject: Re: [PATCH 0/3] Ext3 latency improvement patches

On Fri 27-03-09 17:03:38, Chris Mason wrote:
> 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.  
  Yes.

> 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.
  Yes, this is how we writeout ordered data and obviously it takes long for
such a huge file where you do random IO...

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