From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:52531) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UUvwf-0003Rb-6L for qemu-devel@nongnu.org; Wed, 24 Apr 2013 05:25:02 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UUvwb-0005yZ-SA for qemu-devel@nongnu.org; Wed, 24 Apr 2013 05:24:57 -0400 Received: from mail-qe0-f45.google.com ([209.85.128.45]:37696) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UUvwb-0005yV-Mu for qemu-devel@nongnu.org; Wed, 24 Apr 2013 05:24:53 -0400 Received: by mail-qe0-f45.google.com with SMTP id 1so1048722qee.18 for ; Wed, 24 Apr 2013 02:24:53 -0700 (PDT) Sender: Paolo Bonzini Message-ID: <5177A4D8.3050108@redhat.com> Date: Wed, 24 Apr 2013 11:24:40 +0200 From: Paolo Bonzini MIME-Version: 1.0 References: <5176C33C.6000006@redhat.com> <20130424083758.GB20971@stefanha-thinkpad.redhat.com> In-Reply-To: <20130424083758.GB20971@stefanha-thinkpad.redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] Adding Disk-Level Introspection to QEMU List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Wolfgang Richter , "qemu-devel@nongnu.org" , Wolfgang Richter Il 24/04/2013 10:37, Stefan Hajnoczi ha scritto: >> > Has there been any performance analysis of drive-mirror (impact on executing guest)? What Stefan wrote is about block-backup. drive-mirror has a limited impact on guest performance, but it doesn't pass the writes through to the channel. Instead, it uses a dirty bitmap that it periodically scans to copy new data to the destination. > It slows down guest I/O for a couple of reasons: > > 1. Writes now require a read from the original device followed by a > write to the target device. Only after this completes is the write > allowed to proceed. > > 2. Overlapping read/write requests are serialized to maintain > consistency between the guests I/Os and the block-backup I/Os. > > But on second thought, I don't think block-backup fits the bill. You > don't care about the original data, you care about what new data the > guest is writing. Right. However, when block-backup gets in, I will try to change drive-mirror to use an "active" method. I don't have a timeframe for this, though. Paolo