linux-kernel - Re: Mis-Design of Btrfs?

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Message-Id: <1310735821-sup-2589@shiny>
Date:	Fri, 15 Jul 2011 09:20:00 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	NeilBrown <neilb@...e.de>, david <david@...g.hm>,
	Nico Schottelius <nico-lkml-20110623@...ottelius.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Alasdair G Kergon <agk@...hat.com>
Subject: Re: Mis-Design of Btrfs?

Excerpts from Ric Wheeler's message of 2011-07-15 08:58:04 -0400:
> On 07/15/2011 12:34 PM, Chris Mason wrote:

[ triggering IO retries on failed crc or other checks ]

> >
> > But, maybe the whole btrfs model is backwards for a generic layer.
> > Instead of sending down ios and testing when they come back, we could
> > just set a verification function (or stack of them?).
> >
> > For metadata, btrfs compares the crc and a few other fields of the
> > metadata block, so we can easily add a compare function pointer and a
> > void * to pass in.
> >
> > The problem is the crc can take a lot of CPU, so btrfs kicks it off to
> > threading pools so saturate all the cpus on the box.  But there's no
> > reason we can't make that available lower down.
> >
> > If we pushed the verification down, the retries could bubble up the
> > stack instead of the other way around.
> >
> > -chris
> 
> I do like the idea of having the ability to do the verification and retries down 
> the stack where you actually have the most context to figure out what is possible...
> 
> Why would you need to bubble back up anything other than an error when all 
> retries have failed?

By bubble up I mean that if you have multiple layers capable of doing
retries, the lowest levels would retry first.  Basically by the time we
get an -EIO_ALREADY_RETRIED we know there's nothing that lower level can
do to help.

-chris
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