From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=35009 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OsbOe-00059D-Ro for qemu-devel@nongnu.org; Mon, 06 Sep 2010 09:06:10 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OsbOd-0008BG-8C for qemu-devel@nongnu.org; Mon, 06 Sep 2010 09:06:04 -0400 Received: from mail-gy0-f173.google.com ([209.85.160.173]:51439) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OsbOd-0008B8-4a for qemu-devel@nongnu.org; Mon, 06 Sep 2010 09:06:03 -0400 Received: by gya1 with SMTP id 1so1953121gya.4 for ; Mon, 06 Sep 2010 06:06:02 -0700 (PDT) Message-ID: <4C84E738.3020802@codemonkey.ws> Date: Mon, 06 Sep 2010 08:06:00 -0500 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format References: <1283767478-16740-1-git-send-email-stefanha@linux.vnet.ibm.com> In-Reply-To: <1283767478-16740-1-git-send-email-stefanha@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Kevin Wolf , qemu-devel@nongnu.org On 09/06/2010 05:04 AM, Stefan Hajnoczi wrote: > QEMU Enhanced Disk format is a disk image format that forgoes features > found in qcow2 in favor of better levels of performance and data > integrity. Due to its simpler on-disk layout, it is possible to safely > perform metadata updates more efficiently. > > Installations, suspend-to-disk, and other allocation-heavy I/O workloads > will see increased performance due to fewer I/Os and syncs. Workloads > that do not cause new clusters to be allocated will perform similar to > raw images due to in-memory metadata caching. > > The format supports sparse disk images. It does not rely on the host > filesystem holes feature, making it a good choice for sparse disk images > that need to be transferred over channels where holes are not supported. > > Backing files are supported so only deltas against a base image can be > stored. > > The file format is extensible so that additional features can be added > later with graceful compatibility handling. > > Internal snapshots are not supported. This eliminates the need for > additional metadata to track copy-on-write clusters. > > Compression and encryption are not supported. They add complexity and > can be implemented at other layers in the stack (i.e. inside the guest > or on the host). > > The format is currently functional with the following features missing: > * Resizing the disk image. The capability has been designed in but the > code has not been written yet. > * Resetting the image after backing file commit completes. > * Changing the backing filename. > * Consistency check (fsck). This is simple due to the on-disk layout. > > Signed-off-by: Anthony Liguori > Signed-off-by: Stefan Hajnoczi > Another point worth mentioning is that our intention is to have a formal specification of the format before merging. A start of that is located at http://wiki.qemu.org/Features/QED Regards, Anthony Liguori