Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753892Ab2JYFPH (ORCPT ); Thu, 25 Oct 2012 01:15:07 -0400 Received: from li9-11.members.linode.com ([67.18.176.11]:57389 "EHLO imap.thunk.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751486Ab2JYFPF (ORCPT ); Thu, 25 Oct 2012 01:15:05 -0400 Date: Thu, 25 Oct 2012 01:14:45 -0400 From: "Theodore Ts'o" To: Vladislav Bolkhovitin Cc: =?utf-8?B?5p2o6IuP56uL?= Yang Su Li , General Discussion of SQLite Database , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, drh@hwaci.com Subject: Re: [sqlite] light weight write barriers Message-ID: <20121025051445.GA9860@thunk.org> Mail-Followup-To: Theodore Ts'o , Vladislav Bolkhovitin , =?utf-8?B?5p2o6IuP56uL?= Yang Su Li , General Discussion of SQLite Database , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, drh@hwaci.com References:

<5086F5A7.9090406@vlnb.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5086F5A7.9090406@vlnb.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-SA-Exim-Connect-IP: X-SA-Exim-Mail-From: tytso@thunk.org X-SA-Exim-Scanned: No (on imap.thunk.org); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2150 Lines: 42 On Tue, Oct 23, 2012 at 03:53:11PM -0400, Vladislav Bolkhovitin wrote: > Yes, SCSI has full support for ordered/simple commands designed > exactly for that task: to have steady flow of commands even in case > when some of them are ordered..... SCSI does, yes --- *if* the device actually implements Tagged Command Queuing (TCQ). Not all devices do. More importantly, SATA drives do *not* have this capability, and when you compare the price of SATA drives to uber-expensive "enterprise drives", it's not surprising that most people don't actually use SCSI/SAS drives that have implemented TCQ. SATA's Native Command Queuing (NCQ) is not equivalent; this allows the drive to reorder requests (in particular read requests) so they can be serviced more efficiently, but it does *not* allow the OS to specify a partial, relative ordering of requests. Yes, you can turn off writeback caching, but that has pretty huge performance costs; and there is the FUA bit, but that's just an unconditional high priority bypass of the writeback cache, which is useful in some cases, but which again, does not give the ability for the OS to specify a partial order, while letting the drive reorder other requests for efficiency/performance's sake, since the drive has a lot more information about the optimal way to reorder requests based on the current location of the drive head and where certain blocks may have been remapped due to bad block sparing, etc. > Hopefully, eventually the storage developers will realize the value > behind ordered commands and learn corresponding SCSI facilities to > deal with them. Eventually, drive manufacturers will realize that trying to price guage people who want advanced features such as TCQ, DIF/DIX, is the best way to gaurantee that most people won't bother to purchase them, and hence the features will remain largely unused.... - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/