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authorStefan Hajnoczi <stefanha@linux.vnet.ibm.com>2012-07-27 09:05:18 +0100
committerKevin Wolf <kwolf@redhat.com>2012-08-06 22:39:14 +0200
commit0f6d767aa84676a2374d38797f42df4595415369 (patch)
tree199c11ba0e274a5b93cb1f8b2059ccf60a00b0c8 /docs/specs
parente77964f79b905760ce7f0406fc6b988daeabbb65 (diff)
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 <stefanha@linux.vnet.ibm.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Diffstat (limited to 'docs/specs')
-rw-r--r--docs/specs/qcow2.txt7
1 files changed, 6 insertions, 1 deletions
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