From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1O6fwi-0005hF-Ky for qemu-devel@nongnu.org; Tue, 27 Apr 2010 04:15:08 -0400 Received: from [140.186.70.92] (port=60028 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1O6fwh-0005gV-8J for qemu-devel@nongnu.org; Tue, 27 Apr 2010 04:15:08 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1O6fwf-0006fD-QR for qemu-devel@nongnu.org; Tue, 27 Apr 2010 04:15:07 -0400 Received: from mx1.redhat.com ([209.132.183.28]:47194) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1O6fwf-0006ez-GQ for qemu-devel@nongnu.org; Tue, 27 Apr 2010 04:15:05 -0400 Message-ID: <4BD69D03.2050502@redhat.com> Date: Tue, 27 Apr 2010 11:14:59 +0300 From: Avi Kivity MIME-Version: 1.0 References: <20100426172634.GC15278@x200.localdomain> <4BD5D28C.7080700@codemonkey.ws> <20100426221258.GH15278@x200.localdomain> <4BD61584.9080208@codemonkey.ws> In-Reply-To: <4BD61584.9080208@codemonkey.ws> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] Re: KVM call agenda for Apr 27 List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: Chris Wright , qemu-devel@nongnu.org, kvm@vger.kernel.org On 04/27/2010 01:36 AM, Anthony Liguori wrote: > > A few comments: > > 1) The problem was not block watermark itself but generating a > notification on the watermark threshold. It's a heuristic and should > be implemented based on polling block stats. Polling for an event that never happens is bad engineering. What frequency do you poll? you're forcing the user to make a lose-lose tradeoff. > Otherwise, we'll be adding tons of events to qemu that we'll struggle > to maintain. That's not a valid reason to reject a user requirement. We may argue the requirement is bogus, or that the suggested implementation is wrong and point in a different direction, but saying that we may have to add more code in the future due to other requirements is ... well I can't find a word for it. > > 2) A block plugin doesn't solve the problem if it's just at the > BlockDriverState level because it can't interact with qcow2. Why not? We have a layered model. guest -> qcow2 -> plugin (sends event) -> raw-posix. Just need to insert the plugin at the appropriate layer. > > 3) For general block plugins, it's probably better to tackle userspace > block devices. We have CUSE and FUSE already, a BUSE is a logical > conclusion. We also have an nbd client. Here's another option: an nbd-like protocol that remotes all BlockDriver operations except read and write over a unix domain socket. The open operation returns an fd (SCM_RIGHTS strikes again) that is used for read and write. This can be used to implement snapshots over LVM, for example. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.