linux-kernel - Re: [PATCH 5/5] dm: implement REQ

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Date:	Mon, 16 Aug 2010 15:02:03 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	jaxboe@...ionio.com, linux-fsdevel@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-ide@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
	hch@....de, James.Bottomley@...e.de, tytso@....edu,
	chris.mason@...cle.com, swhiteho@...hat.com,
	konishi.ryusuke@....ntt.co.jp, dm-devel@...hat.com, vst@...b.net,
	jack@...e.cz, rwheeler@...hat.com, hare@...e.de, neilb@...e.de,
	rusty@...tcorp.com.au, mst@...hat.com, Tejun Heo <tj@...nle.org>
Subject: Re: [PATCH 5/5] dm: implement REQ_FLUSH/FUA support

On Mon, Aug 16 2010 at 12:52pm -0400,
Tejun Heo <tj@...nel.org> wrote:

> From: Tejun Heo <tj@...nle.org>
> 
> This patch converts dm to support REQ_FLUSH/FUA instead of now
> deprecated REQ_HARDBARRIER.

What tree does this patch apply to?  I know it doesn't apply to
v2.6.36-rc1, e.g.: http://git.kernel.org/linus/708e929513502fb0

> For bio-based dm,
...
> * -EOPNOTSUPP retry logic dropped.

That logic wasn't just about retries (at least not in the latest
kernel).  With commit 708e929513502fb0 the -EOPNOTSUPP checking also
serves to optimize the barrier+discard case (when discards aren't
supported).

> For request-based dm,
> 
> * Nothing much changes.  It just needs to handle FLUSH requests as
>   before.  It would be beneficial to advertise FUA capability so that
>   it can propagate FUA flags down to member request_queues instead of
>   sequencing it as WRITE + FLUSH at the top queue.

Can you expand on that TODO a bit?  What is the mechanism to propagate
FUA down to a DM device's members?  I'm only aware of propagating member
devices' features up to the top-level DM device's request-queue (not the
opposite).

Are you saying that establishing the FUA capability on the top-level DM
device's request_queue is sufficient?  If so then why not make the
change?

> Lightly tested linear, stripe, raid1, snap and crypt targets.  Please
> proceed with caution as I'm not familiar with the code base.

This is concerning... if we're to offer more comprehensive review I
think we need more detail on what guided your changes rather than
details of what the resulting changes are.

Mike
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