| Message ID | 1340377726-5896-2-git-send-email-stefanha@linux.vnet.ibm.com |
|---|---|
| State | New |
| Headers | show |
diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt index 87bf785..3a789ce 100644 --- a/docs/specs/qcow2.txt +++ b/docs/specs/qcow2.txt @@ -75,7 +75,11 @@ 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
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> --- docs/specs/qcow2.txt | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)