From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with ESMTP id 609606B00A6 for ; Wed, 29 Jul 2009 17:18:43 -0400 (EDT) Date: Wed, 29 Jul 2009 23:18:45 +0200 From: Jens Axboe Subject: Re: Why does __do_page_cache_readahead submit READ, not READA? Message-ID: <20090729211845.GB4148@kernel.dk> References: <20090729161456.GB8059@barkeeper1-xen.linbit> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090729161456.GB8059@barkeeper1-xen.linbit> Sender: owner-linux-mm@kvack.org To: Lars Ellenberg Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, dm-devel@redhat.com, Neil Brown List-ID: 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org