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docs: add dirty bit to qcow2 specification

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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>
This commit is contained in:
Stefan Hajnoczi 2012-07-27 09:05:18 +01:00 committed by Kevin Wolf
parent e77964f79b
commit 0f6d767aa8
1 changed files with 6 additions and 1 deletions
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7
docs/specs/qcow2.txt
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@ -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