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

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Date:	Wed, 29 Jul 2009 18:55:01 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Lars Ellenberg <lars.ellenberg@...bit.com>,
	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 at 11:18:45PM +0200, Jens Axboe wrote:
> 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.

One of the problems in the past was that reada would fail if there
wasn't a free request when we actually wanted it to go ahead and wait.
Or something.  We've switched it around a few times I think.

-chris

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