From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jens Axboe Subject: Re: Servicing of requests Date: Sun, 23 Mar 2003 13:28:10 +0100 Sender: linux-scsi-owner@vger.kernel.org Message-ID: <20030323122810.GC2371@suse.de> References: <20030323121850.48607.qmail@mail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20030323121850.48607.qmail@mail.com> List-Id: linux-scsi@vger.kernel.org To: Subodh S Cc: linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org On Sun, Mar 23 2003, Subodh S wrote: > Hi, > > Whenever I read data of 'x'k size using one read() system call, I find > batches of some 'y' no. of make_requests calls followed by the same > no. of end_io's. Something like : > make_req > make_req > make_req > end_io > end_io > end_io > make_req > make_req > make_req > end_io > end_io > end_io > > The output above gives me an idea that 3(hypothetical no.) > buffer_heads above form a request. (since 1 make_request corresponds > to 1 buffer_head) and maybe since 1 request is serviced at a time I > can see 3 make_req's together. Is my understanding right ?? > > But, I have read that sd uses some optimization algorithm to club > requests so that the disk seek time is reduced. In which case since > all requests are to adjecents sectors it should create a single > request of all 'x'k assuming 1 buffer_head is of size 1k. > > Does this make sense ?? First of all, please line wrap your emails at 72 chars. Your mail reads horribly. Second, what is your question? Yes typically buffer_heads can get clustered into a request so that contig regions on disk are handed to the driver as a single request that may contain X buffer_heads. sd doesn't do this on its own, the block layer does it for the driver. And it happens for all drivers. -- Jens Axboe