From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:32922) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WJ3a4-00034a-L6 for qemu-devel@nongnu.org; Thu, 27 Feb 2014 11:13:16 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1WJ3Zv-0006ht-Bc for qemu-devel@nongnu.org; Thu, 27 Feb 2014 11:13:04 -0500 Received: from mx.ipv6.kamp.de ([2a02:248:0:51::16]:44082 helo=mx01.kamp.de) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WJ3Zv-0006hO-0q for qemu-devel@nongnu.org; Thu, 27 Feb 2014 11:12:55 -0500 Message-ID: <530F6401.8020501@kamp.de> Date: Thu, 27 Feb 2014 17:12:49 +0100 From: Peter Lieven MIME-Version: 1.0 References: <530DBE6C.5030502@kamp.de> <20140226154154.GB20820@stefanha-thinkpad.muc.redhat.com> <20140227011031.GC1050@T430> <20140227110741.GC3667@dhcp-200-207.str.redhat.com> In-Reply-To: <20140227110741.GC3667@dhcp-200-207.str.redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] qemu-img convert cache mode for source List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf , Fam Zheng Cc: Stefan Hajnoczi , "qemu-devel@nongnu.org" , Stefan Hajnoczi , Paolo Bonzini Am 27.02.2014 12:07, schrieb Kevin Wolf: > Am 27.02.2014 um 02:10 hat Fam Zheng geschrieben: >> On Wed, 02/26 16:41, Stefan Hajnoczi wrote: >>> On Wed, Feb 26, 2014 at 11:14:04AM +0100, Peter Lieven wrote: >>>> I was wondering if it would be a good idea to set the O_DIRECT mode for the source >>>> files of a qemu-img convert process if the source is a host_device? >>>> >>>> Currently the backup of a host device is polluting the page cache. >> Peter, can you give some more detailed explanation of the issue? An example or >> benchmark will help a lot. As I understand, one time scanning of a file doesn't >> promote the page cache to active list, so it probably won't hurt real hot cache >> at all, and will get replaced very soon. >> >> Considering readahead and page cache on metadata, I'm not sure if forcing >> O_DIRECT is a good idea. >> >>> The problem is what to do for image formats. An image file can be >>> very fragmented so the readahead might not be a win. Does this mean >>> that for image formats we should tell the kernel access will be >>> random? >>> >>> Furthermore, maybe it's best to do readahead inside QEMU so that even >>> network protocols (nbd, iscsi, etc) can get good performance. They >>> act like O_DIRECT is always on. >> Also, experience with booting a network backed guest can be greatly improved, >> because sometimes BIOS and bootloader are simple minded and load a kernel or >> initrd by sending thousands of 1 sector requests with iodepth=1, which means >> latency of network based IO hurts a lot. > I think I mentioned it a while ago, but our IDE emulation plays a role > in this as well. PIO requests are always handled sector by sector, no > matter how big the request was that we got from the BIOS. Yes, you have pointed that out before. How complicated is it to fix this? Peter > > Kevin > >> Doing readahead in QEMU makes sense for image formats as well, because we know >> where the next data cluster is better than kernel. But again, we are >> replicating things from kernel.