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[Qemu-devel] [PATCH v3 1/7] docs: add dirty bit to qcow2 specification

From: Stefan Hajnoczi
Subject: [Qemu-devel] [PATCH v3 1/7] docs: add dirty bit to qcow2 specification
Date: Fri, 27 Jul 2012 09:05:18 +0100

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 <address@hidden>
 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 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

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