From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:60091) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SyUAh-0008Ir-CX for qemu-devel@nongnu.org; Mon, 06 Aug 2012 16:45:04 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1SyUAf-0004Ox-Nn for qemu-devel@nongnu.org; Mon, 06 Aug 2012 16:45:03 -0400 Received: from mx1.redhat.com ([209.132.183.28]:22686) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SyUAf-0004OK-GD for qemu-devel@nongnu.org; Mon, 06 Aug 2012 16:45:01 -0400 From: Kevin Wolf Date: Mon, 6 Aug 2012 22:44:43 +0200 Message-Id: <1344285891-6578-5-git-send-email-kwolf@redhat.com> In-Reply-To: <1344285891-6578-1-git-send-email-kwolf@redhat.com> References: <1344285891-6578-1-git-send-email-kwolf@redhat.com> Subject: [Qemu-devel] [PATCH 04/12] docs: add dirty bit to qcow2 specification List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: anthony@codemonkey.ws Cc: kwolf@redhat.com, qemu-devel@nongnu.org From: Stefan Hajnoczi The dirty bit will make it possible to perform lazy refcount updates, where the image file is not kept consistent all the time. Upon opening a dirty image file, it is necessary to perform a consistency check and repair any incorrect refcounts. Therefore the dirty bit must be an incompatible feature bit. We don't want old programs accessing a file with stale refcounts. Signed-off-by: Stefan Hajnoczi Signed-off-by: Kevin Wolf --- docs/specs/qcow2.txt | 7 ++++++- 1 files changed, 6 insertions(+), 1 deletions(-) diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt index 87bf785..339cdc1 100644 --- a/docs/specs/qcow2.txt +++ b/docs/specs/qcow2.txt @@ -75,7 +75,12 @@ in the description of a field. Bitmask of incompatible features. An implementation must fail to open an image if an unknown bit is set. - Bits 0-63: Reserved (set to 0) + Bit 0: Dirty bit. If this bit is set then refcounts + may be inconsistent, make sure to scan L1/L2 + tables to repair refcounts before accessing the + image. + + Bits 1-63: Reserved (set to 0) 80 - 87: compatible_features Bitmask of compatible features. An implementation can -- 1.7.6.5