From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bart Van Assche Subject: Re: SCSI mid layer and high IOPS capable devices Date: Fri, 14 Dec 2012 17:15:37 +0100 Message-ID: <50CB50A9.1050202@acm.org> References: <20121211000013.GI23107@beardog.cce.hp.com> <50C9F2B9.4050500@acm.org> <20121213172513.GH20898@beardog.cce.hp.com> <50CA0692.2010903@acm.org> <20121213164912.GA28496@infradead.org> <50CAF502.3070300@acm.org> <20121214164421.GQ20898@beardog.cce.hp.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from georges.telenet-ops.be ([195.130.137.68]:47894 "EHLO georges.telenet-ops.be" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756508Ab2LNQPi (ORCPT ); Fri, 14 Dec 2012 11:15:38 -0500 In-Reply-To: <20121214164421.GQ20898@beardog.cce.hp.com> Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: scameron@beardog.cce.hp.com Cc: Christoph Hellwig , linux-scsi@vger.kernel.org, stephenmcameron@gmail.com, dab@hp.com On 12/14/12 17:44, scameron@beardog.cce.hp.com wrote: > I expect the flash devices re-order requests as well, simply because > to feed requests to the things at a sufficient rate, you have to pump > requests into them concurrently on multiple hardware queues -- a single > cpu jamming requests into them as fast as it can is still not fast enough > to keep them busy. Consequently, they *can't* care about ordering, as the > relative order requests on different hardware queues are submitted into them > is not even really controlled, so the OS *can't* count on concurrent requests > not to be essentially "re-ordered", just because of the nature of the way > requests get into the device. Why should a flash device have to reorder write requests ? These devices typically use a log-structured file system internally. Bart.