The lazy refcounts bit indicates that this image can take advantage of
the dirty bit and that refcount updates can be postponed.
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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 updates the qcow2 specification to cover version 3. It contains the
following changes:
- Added compatible/incompatible/auto-clear feature bits plus an optional
feature name table to allow useful error messages even if an older
version doesn't know some feature at all.
- Configurable refcount width. If you don't want to use internal
snapshots, make refcounts one bit and save cache space and I/O.
- Zero cluster flags. This allows discard even with a backing file that
doesn't contain zeros. It is also useful for copy-on-read/image
streaming, as you'll want to keep sparseness without accessing the
remote image for an unallocated cluster all the time.
- Fixed internal snapshot metadata to use 64 bit VM state size. You
can't save a snapshot of a VM with >= 4 GB RAM today.
- Extended internal snapshot metadata to contain the disk size, so that
resizing images that have snapshots can be allowed in the future.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This adds a description of the qcow2 file format to the docs/ directory.
Besides documenting what's there, which is never wrong, the document should
provide a good basis for the discussion of format extensions (called "qcow3"
in previous discussions)
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
