linux-ext4 - Re: [PATCH 1/2] HACK: ext3: mount fast even when recovering

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Date:	Wed, 15 Jul 2009 18:35:34 +0300
From:	Adrian Hunter <adrian.hunter@...ia.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	"sct@...hat.com" <sct@...hat.com>,
	"adilger@....com" <adilger@....com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"Bityutskiy Artem (Nokia-D/Helsinki)" <Artem.Bityutskiy@...ia.com>
Subject: Re: [PATCH 1/2] HACK: ext3: mount fast even when recovering

Andrew Morton wrote:
> On Tue, 14 Jul 2009 17:05:54 +0300
> Adrian Hunter <adrian.hunter@...ia.com> wrote:
> 
>> Speed up ext3 recovery mount time by not sync'ing the
>> block device.  Instead place all dirty buffers into the
>> I/O queue and add a write barrier.  This ensures that
>> no subsequent write will reach the disk before all the
>> recovery writes, but that we do not have to wait for the
>> I/O.
>>
>> Note that ext3 reads sectors the correct way: through the
>> buffer cache, so there is no risk of reading old metadata.
> 
> hm.  The change seems reasonable to me.  afaict it leaves no timing
> windows during which another crash could muck things up.
> 
> As long as those write barriers actually work.  Do they?  For all
> conceivable devices and IO schedulers?

As far as I know I/O barriers work.  The I/O schedulers are forcibly
drained so they do not affect the barrier.  I am not sure about all
devices - I guess some device drivers might return errors if asked to
provide a barrier and they can't.

Our device is a MMC and does one I/O at a time, so hardware barriers
are not needed and are ignored.

> It would be useful if you could quantify the benefits please - some
> before-and-after timing results with both your funky hardware as well
> as regular old disks would suit.

I will send some examples.

> I'd suggest that if we're going to do this, we should aim to do it
> unconditionally - no mount option needed.  We could leave the option
> there for a while, for testing purposes (ie: we think the code might be
> buggy).  But the new feature should perhaps default to "on", and we
> plan to remove the mount option after a while.
> 
> Because there's no reason to retain the mount option in the long term.

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