From 0f6d767aa84676a2374d38797f42df4595415369 Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Fri, 27 Jul 2012 09:05:18 +0100 Subject: [PATCH] docs: add dirty bit to qcow2 specification 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 file changed, 6 insertions(+), 1 deletion(-) diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt index 87bf785fe0..339cdc1b58 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