From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:48118) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QCpan-0002rh-5C for qemu-devel@nongnu.org; Thu, 21 Apr 2011 04:50:30 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1QCpam-0007mi-69 for qemu-devel@nongnu.org; Thu, 21 Apr 2011 04:50:29 -0400 Received: from verein.lst.de ([213.95.11.211]:53455 helo=newverein.lst.de) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QCpal-0007mc-Uw for qemu-devel@nongnu.org; Thu, 21 Apr 2011 04:50:28 -0400 Date: Thu, 21 Apr 2011 10:50:27 +0200 From: Christoph Hellwig Message-ID: <20110421085026.GA4817@lst.de> References: <1303375391-26679-1-git-send-email-ronniesahlberg@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1303375391-26679-1-git-send-email-ronniesahlberg@gmail.com> Subject: Re: [Qemu-devel] iSCSI support for QEMU List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Ronnie Sahlberg Cc: qemu-devel@nongnu.org, stefanha@linux.vnet.ibm.com On Thu, Apr 21, 2011 at 06:43:10PM +1000, Ronnie Sahlberg wrote: > Some basic tests thatve been performed show it to be significantly faster > than an out-of-the-box open-iscsi mounted LUN being accessed by default > QEMU i/o options. Which isn't a useful comparism. qemu's default is the braindead cache=writethrough behaviour, which forces writes out to disk, and bloats the pagecache, while your mail sounds you silently always implement O_DIRECT-like semantics. Also the implementation has data integrity issues due this. It does not actually force writes out to disk in the default cache mode, despite qemu claiming to have WCE=0 in the default mode, i.e. you never flush the cache. You'll need set the FUA bits on all writes if cache=writethrough is used, with a fallback to a real cache flush in case the target doesn't support the FUA bit.