From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=58441 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1ON0dC-00023Y-Vd for qemu-devel@nongnu.org; Fri, 11 Jun 2010 05:34:32 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1ON0dB-0005ud-PZ for qemu-devel@nongnu.org; Fri, 11 Jun 2010 05:34:30 -0400 Received: from mx1.redhat.com ([209.132.183.28]:29037) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1ON0dB-0005uE-Ii for qemu-devel@nongnu.org; Fri, 11 Jun 2010 05:34:29 -0400 Message-ID: <4C120311.8080406@redhat.com> Date: Fri, 11 Jun 2010 11:34:09 +0200 From: Kevin Wolf MIME-Version: 1.0 Subject: Re: [Qemu-devel] [Bug 322602] Re: Snapshot usage makes qcow2 image unusable due to large tables References: <20090129032712.7039.50315.malonedeb@gandwana.canonical.com> <20100611075959.13068.85161.malone@soybean.canonical.com> In-Reply-To: <20100611075959.13068.85161.malone@soybean.canonical.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Bug 322602 <322602@bugs.launchpad.net> Cc: Jes Sorensen , qemu-devel@nongnu.org Am 11.06.2010 09:59, schrieb Jes Sorensen: > Could you please let us know whether this is still a problem and if it > isn't, lets close this bug. This looks like a purely hypothetical thing - have you seen this happen in reality, and if so, with how many snapshots? This is reported against a very old version, so we have to assume 4k clusters. This means that a refcount block holds the refcounts for 4k / 2 = 2k clusters. Let's assume a refcount table of only one cluster, so we can describe (4k / 8) * 2k = 1M clusters with this, which makes up an image size of 4G. To hold a 20G virtual disk plus some metadata we'll therefore need something like 6 clusters = 24k. To make the refcount table consume just 1 MB, you'll therefore need at least 42 snapshots, each fully allocated on its own, consuming 1 TB for the image. I doubt that there are too many machines which can handle a 1 TB image file on disk, but not a 1 MB refcount table in RAM. Nowadays, of course, we're using 64k clusters by default. With a refcount table of 64k we describe 16 TB there, with a 1 MB refcount table it's 256 TB. Kevin