From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1Jyqlz-0006BU-9o for qemu-devel@nongnu.org; Wed, 21 May 2008 12:02:39 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1Jyqly-0006AT-B3 for qemu-devel@nongnu.org; Wed, 21 May 2008 12:02:38 -0400 Received: from [199.232.76.173] (port=44524 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1Jyqly-0006AF-3g for qemu-devel@nongnu.org; Wed, 21 May 2008 12:02:38 -0400 Received: from py-out-1112.google.com ([64.233.166.182]:47597) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1Jyqlx-0000o1-Oi for qemu-devel@nongnu.org; Wed, 21 May 2008 12:02:37 -0400 Received: by py-out-1112.google.com with SMTP id p76so222620pyb.10 for ; Wed, 21 May 2008 09:02:35 -0700 (PDT) Message-ID: <48344793.2020902@codemonkey.ws> Date: Wed, 21 May 2008 11:02:27 -0500 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT) References: <200805210205.37432.paul@codesourcery.com> <4833778C.4030209@codemonkey.ws> <4833DC3F.8000604@suse.de> <20080521122629.GA14416@shareable.org> <48341783.3060204@qumranet.com> <20080521134154.GA15210@shareable.org> <483429EB.7070705@codemonkey.ws> <48342F05.2090603@qumranet.com> <48343106.4070801@codemonkey.ws> <48343844.1050107@qumranet.com> <20080521153454.GB20527@shareable.org> In-Reply-To: <20080521153454.GB20527@shareable.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Reply-To: qemu-devel@nongnu.org List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jamie Lokier Cc: Blue Swirl , Laurent Vivier , qemu-devel@nongnu.org, Paul Brook Jamie Lokier wrote: > Avi Kivity wrote: > >> Here's a summary of the use cases I saw so far: >> >> - casual use, no critical data: write back cache >> >> - backing file shared among many guests: read-only, cached >> >> - desktop system, but don't lose my data: O_SYNC >> (significant resources on the host) >> >> - dedicated virtualization engine: O_DIRECT >> (most host resources assigned to guests) >> > > Sounds alright, but on _my_ desktop system (a laptop), I would use O_DIRECT. > > There isn't enough RAM in my system to be happy duplicating data in > guests and hosts at the same time. VMs are quite demanding on RAM. > > However, if you find a way to map host cached pages into the guest > without copying - so sharing the RAM - that would be excellent. It > can be done in principle, by remapping pages to satisfy IDE/SCSI DMA > requests. I don't know if it would be fast enough. Perhaps it would > work better in KVM than QEMU. > Actually, this is precisely what I'd like to do. The key is to mmap(MAP_PRIVATE) from the underling file directly into the guest's memory. Should be just as applicable to QEMU as KVM (although for KVM we need mmu-notifiers first). Should be a pretty good win when running multiple guests with the same backing file too. Regards, Anthony Liguori > -- Jamie >