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| author | Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> | 2012-07-27 09:05:18 +0100 |
|---|---|---|
| committer | Kevin Wolf <kwolf@redhat.com> | 2012-08-06 22:39:14 +0200 |
| commit | 0f6d767aa84676a2374d38797f42df4595415369 (patch) | |
| tree | 199c11ba0e274a5b93cb1f8b2059ccf60a00b0c8 /docs/specs | |
| parent | e77964f79b905760ce7f0406fc6b988daeabbb65 (diff) | |
| download | qemu-0f6d767aa84676a2374d38797f42df4595415369.tar.gz qemu-0f6d767aa84676a2374d38797f42df4595415369.tar.bz2 qemu-0f6d767aa84676a2374d38797f42df4595415369.zip | |
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.txt | 7 |
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 |