linux-kernel - Re: Why does __do_page_cache_readahead submit READ, not READA?

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Message-ID: <20090729211845.GB4148@kernel.dk>
Date:	Wed, 29 Jul 2009 23:18:45 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Lars Ellenberg <lars.ellenberg@...bit.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, dm-devel@...hat.com,
	Neil Brown <neilb@...e.de>
Subject: Re: Why does __do_page_cache_readahead submit READ, not READA?

On Wed, Jul 29 2009, Lars Ellenberg wrote:
> I naively assumed, from the "readahead" in the name, that readahead
> would be submitting READA bios. It does not.
> 
> I recently did some statistics on how many READ and READA requests
> we actually see on the block device level.
> I was suprised that READA is basically only used for file system
> internal meta data (and not even for all file systems),
> but _never_ for file data.
> 
> A simple
> 	dd if=bigfile of=/dev/null bs=4k count=1
> will absolutely cause readahead of the configured amount, no problem.
> But on the block device level, these are READ requests, where I'd
> expected them to be READA requests, based on the name.
> 
> This is because __do_page_cache_readahead() calls read_pages(),
> which in turn is mapping->a_ops->readpages(), or, as fallback,
> mapping->a_ops->readpage().
> 
> On that level, all variants end up submitting as READ.
> 
> This may even be intentional.
> But if so, I'd like to understand that.

I don't think it's intentional, and if memory serves, we used to use
READA when submitting read-ahead. Not sure how best to improve the
situation, since (as you describe), we lose the read-ahead vs normal
read at that level. I did some experimentation some time ago for
flagging this, see:

http://git.kernel.dk/?p=linux-2.6-block.git;a=commitdiff;h=16cfe64e3568cda412b3cf6b7b891331946b595e

which should pass down READA properly.

-- 
Jens Axboe

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