linux-kernel - Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush

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Date:	Mon, 23 Aug 2010 14:36:37 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Hellwig <hch@....de>
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,
	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
Subject: Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with
 sequenced flush

Hello,

On 08/20/2010 03:22 PM, Christoph Hellwig wrote:
> Many storage devices, especially in the consumer market, come with volatile
> write back caches.  That means the devices signal I/O completion to the
> operating system before data actually has hit the physical medium.

A bit nit picky but flash devices can also have writeback caches and
the term physical medium sounds a bit off for those cases.  Maybe just
saying "non-volatile media" is better?

> Implementation details for filesystems
> --------------------------------------
> 
> Filesystem can simply set the REQ_FLUSH and REQ_FUA bits and do not have to
> worry if the underlying devices need any explicit cache flushing and how
> the Forced Unit Access is implemented.  The REQ_FLUSH and REQ_FUA flags
> may both be set on a single bio.

It may be worthwhile to explain the sequence of operations when
REQ_FLUSH + data + REQ_FUA is executed.  It can be extrapolated from
the previous two descriptions but I think giving examples of different
sequences depending on FLUSH/FUA configuration would be helpful to
help understanding the overall picture of things.

Other than those, looks good to me.

Thanks.

-- 
tejun
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