linux-ext4 - Re: [PATCH] Add ext3 data=guarded mode

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Date:	Mon, 20 Apr 2009 18:26:53 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	Mike Galbraith <efault@....de>
Cc:	Jan Kara <jack@...e.cz>, Theodore Tso <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] Add ext3 data=guarded mode

On (Mon) Apr 20 2009 [14:15:14], Mike Galbraith wrote:
> On Mon, 2009-04-20 at 11:26 +0200, Jan Kara wrote:
> > On Mon 20-04-09 11:07:17, Mike Galbraith wrote:
> > > All journal modes seem subject to bad throughput under heavy pressure,
> > > though data=ordered seems much less likely to suffer for some reason.
> > > Major difference _seems_ to be that write()+largefile induces very much
> > > swap activity.
> 
> >   My rough guess is that this depends on the VM writeout behavior. In
> > ordered mode, we forcibly writeout all the dirty data on a transaction
> > commit which happens every 5 seconds so they don't accumulate that much.
> 
> Aha.
> 
> > In other journaling modes we don't do that and decisions about writeout
> > (probably how much pdflush manages to write in background vs. how much
> > VM throttles the process to do the writeback itself) cause variances in
> > the run time. But this is just a guess. You could gather blktraces of
> > slow and fast runs and then look if the amount of IO done by different
> > processes significantly differs. If Chris has merged by improvements to
> > Seekwatcher, then you could nicely visualize this (hmm, that doesn't seem
> > to be the case so I'm attaching the diff and a helper script - see comments
> > in the beginning of the script and command helps for usage).
> 
> Not necessary methinks.  Actually _reading_ the proggy instead of just
> glancing at it...
> 
> @@ -178,7 +178,7 @@ int do_write_chunks(char *target, int *f
>         unsigned long long remain = len;
>         char *zeros;
> 
> -       zeros = calloc(1, len);
> +       zeros = calloc(1, data);
> 
> I assume that was a booboo. 

Ouch! thanks for spotting that!

I'll re-run the tests later tonight..

		Amit
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