From mboxrd@z Thu Jan 1 00:00:00 1970 From: Steve Ofsthun Subject: Re: Performance on QEMU IDE disks Date: Fri, 14 Jul 2006 15:41:23 -0400 Message-ID: <44B7F363.1050106@virtualiron.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: "P M, Priya (STSD)" Cc: xen-devel@lists.xensource.com List-Id: xen-devel@lists.xenproject.org P M, Priya (STSD) wrote: > We have disabled the Write Cache using sdparm and hdparm utilities. But > if you see the results in Linux Full Virtualization, the Sequential > Write IOPs are better than Native performance. I am wondering how is > this possible? Is there anywhere the caching is happening in the full > virtualization case? If you have an idea about this behavior, please let > me know. QEMU does indeed use the buffer cache in Domain 0. In doing so, it also will take advantage of Domain 0 read ahead/write behind. The down side is that the disk write ordering guarantees expected by DomU filesystems are violated as well. If XEN or Domain 0 crashes, your DomU filesystems may be toast. To fix this you would need to patch QEMU to used O_DIRECT when accessing the virtual disk backing store object (block device, file, etc). We are currently testing just such a patch for the old QEMU device model. I haven't looked at the new device model to see if it already handles this. Steve -- Steve Ofsthun - Virtual Iron Software, Inc.