linux-kernel - Re: Performance testing of various barrier reduction patches [was: Re: [RFC v4] ext4: Coordinate fsync requests]

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Message-ID: <20101019182817.GA11810@lst.de>
Date:	Tue, 19 Oct 2010 20:28:18 +0200
From:	Christoph Hellwig <hch@....de>
To:	"Darrick J. Wong" <djwong@...ibm.com>
Cc:	Andreas Dilger <adilger.kernel@...ger.ca>,
	"Ted Ts'o" <tytso@....edu>, Mingming Cao <cmm@...ibm.com>,
	Ric Wheeler <rwheeler@...hat.com>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Keith Mannthey <kmannth@...ibm.com>,
	Mingming Cao <mcao@...ibm.com>, Tejun Heo <tj@...nel.org>,
	hch@....de
Subject: Re: Performance testing of various barrier reduction patches [was: Re: [RFC v4] ext4: Coordinate fsync requests]

On Mon, Oct 18, 2010 at 03:49:36PM -0700, Darrick J. Wong wrote:
> Through this table, I'm looking for a performance characteristic that typifies
> storage with a battery-backed write cache (BBWC).  As we can see from
> lldd_flush_rtt_avg, the BBWC storage features a very low flush time, about 1ms
> or less.  Everything else, including SSDs, are over that amount.  The other odd
> result I see is that it takes a significant amount of time to get a flush
> command from the top of the block layer to the LLDD, though I suspect some of
> that might be waiting for the device to process earlier writes.  Christoph has
> a patch that looks like it streamlines that, but it triggered various BUG_ONs
> when I booted with it, so I took the patch out.

We currently synchronize flush requests.  There's no real reason to do
it except that we'll either need to make drivers accept flush requests
with a bio attached to them or find a workaround in the block layer to
submit it without bio without synchronizing them.

I thin kthat should be the first angle of attack before adding
complexity to filesystems.

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