linux-kernel - Re: [PATCH 3/3] block: reimplement FLUSH/FUA to support merge

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Date:	Sun, 23 Jan 2011 11:29:20 +0100
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	axboe@...nel.dk, tytso@....edu, djwong@...ibm.com, shli@...nel.org,
	neilb@...e.de, adilger.kernel@...ger.ca, jack@...e.cz,
	snitzer@...hat.com, linux-kernel@...r.kernel.org,
	kmannth@...ibm.com, cmm@...ibm.com, linux-ext4@...r.kernel.org,
	rwheeler@...hat.com, hch@....de, josef@...hat.com
Subject: Re: [PATCH 3/3] block: reimplement FLUSH/FUA to support merge

On Sun, Jan 23, 2011 at 11:25:26AM +0100, Tejun Heo wrote:
> Again, issuing flushes as fast as possible isn't necessarily better.
> It might feel counter-intuitive but it generally makes sense to delay
> flush if there are a lot of concurrent flush activities going on.
> Another related interesting point is that with flush merging,
> depending on workload, there's a likelihood that FUA, even if the
> device supports it, might result in worse performance than merged DATA
> + single POSTFLUSH sequence.

Let me add a bit.

In general, I'm a bit skeptical about the usefulness of hardware FUA
on a rotating disk.  All it saves is a single command issue overhead.
On storage array or SSDs, the balance might be different tho.  Event
hen, with flush merging, I think it would heavily depend on the
workload which way it would turn out.

Thanks.

-- 
tejun
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