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

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Date:	Fri, 27 Mar 2009 17:03:38 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	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, 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


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