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

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Date:	Fri, 15 Jul 2011 14:31:37 +0100
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Chris Mason <chris.mason@...cle.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?

On 07/15/2011 02:20 PM, Chris Mason wrote:
> 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

Absolutely sounds like the most sane way to go to me, thanks!

Ric

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