From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ric Wheeler Subject: Re: [dm-devel] [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush Date: Mon, 23 Aug 2010 12:49:54 -0400 Message-ID: <4C72A6B2.4070900@redhat.com> References: <1281616891-5691-1-git-send-email-tj@kernel.org> <20100820132214.GA6184@lst.de> <4C6E9CAF.5010202@redhat.com> <4C7269E9.9070304@kernel.org> <20100823124815.GA20095@lst.de> <4C727E96.5020801@redhat.com> <4C727F2B.6060501@fusionio.com> <4C729171.3030605@redhat.com> <20100823164522.GA9528@atlas.home> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from mx1.redhat.com ([209.132.183.28]:39330 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752730Ab0HWQt6 (ORCPT ); Mon, 23 Aug 2010 12:49:58 -0400 In-Reply-To: <20100823164522.GA9528@atlas.home> Sender: linux-ide-owner@vger.kernel.org List-Id: linux-ide@vger.kernel.org To: Jens Axboe , "tytso@mit.edu" , "linux-scsi@vger.kernel.org" , "linux-ide@vger.kernel.org" On 08/23/2010 12:45 PM, Sergey Vlasov wrote: > On Mon, Aug 23, 2010 at 11:19:13AM -0400, Ric Wheeler wrote: > [...] >> (2) hardware raid cards with internal buffer memory and on-card battery backup >> (they sit in your server, disks sit in jbod like expansion shelves). These are >> fine if the drives in those shelves have write cache disabled. > > Actually some of such cards keep write cache on the drives enabled and > issue FLUSH CACHE commands to the drives. E.g., 3ware 9690SA behaves > like this at least with SATA drives (the FLUSH CACHE commands can be > seen after enabling performance monitoring - they often end up in the > "10 commands having the largest latency" table). This can actually be > safe if the card waits for the FLUSH CACHE completion before making > the write cache data in its battery-backed memory available for reuse > (and the drive implements the FLUSH CACHE command correctly). Yes - this is certainly one way to do it. Note that this will not work if the card advertises itself as a write through cache (and we end up not sending down the SYNC_CACHE commands). At least one hardware RAID card (I unfortunately cannot mention the brand) did not do this command forwarding. ric