linux-ext4 - Re: What happened to data=guarded?

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Date:	Wed, 12 Aug 2009 22:37:56 +0200
From:	Jan Kara <jack@...e.cz>
To:	Pavel Machek <pavel@....cz>
Cc:	Theodore Tso <tytso@....edu>, Andi Kleen <andi@...stfloor.org>,
	Frans Pop <elendil@...net.nl>,
	Chris Mason <chris.mason@...cle.com>, jack@...e.cz,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: What happened to data=guarded?

On Mon 10-08-09 18:42:05, Pavel Machek wrote:
> On Tue 2009-08-11 14:57:03, Theodore Tso wrote:
> > On Tue, Aug 11, 2009 at 05:29:14PM +0200, Andi Kleen wrote:
> > > > Good to hear. I've so far stayed with data=ordered as I think I'd prefer 
> > > > data=guarded over data=writeback. I'll certainly give it a try when it's 
> > > > available.
> > > 
> > > Same here. data=writeback already cost me a few files after crashes here :/
> > 
> > What sort of files were you losing?  I don't know if we can improve
> > the implied flush hueristics, but we should at least try to see if we
> > do something about it.
> 
> IIRC... the flush heuristics invoke async flush, so you can still lose
> data if you are unlucky, no?
  Of course you can but it can happen in data=ordered mode as well (if the
machine crashes before the transaction is committed). The percieved
difference is in the fact that kjournald starts its commit every 5 seconds
while pdflush starts writeback every 30-35 seconds. So if you use
data=guarded/writeback mode and set dirty_expire_centisecs to 500, the
experience wrt. data loss is going to be similar to data=ordered mode.
fsync with heavy background writers won't be that painful as in data=ordered
mode but apart from that the performance will be probably comparable.

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