Am 29.08.2013 um 14:52 hat Max Reitz geschrieben: > Am 29.08.2013 14:45, schrieb Eric Blake: > >On 08/29/2013 05:20 AM, Max Reitz wrote: > >>Implement bdrv_amend_options for compat, size, backing_file, backing_fmt > >>and lazy_refcounts. > >> > >>Downgrading images from compat=1.1 to compat=0.10 is achieved through > >>handling all incompatible flags accordingly, clearing all compatible and > >>autoclear flags and expanding all zero clusters. > >> > >>Signed-off-by: Max Reitz <mre...@redhat.com> > >>--- > >>+/* > >>+ * Expands all zero clusters on the image; important for downgrading to a > >>qcow2 > >>+ * version which doesn't yet support metadata zero clusters. > >Do we have to fully write 0 blocks into the image no matter what, or are > >there cases where, when the file has a backing image and we know the > >backing image has 0 bytes at the same offset, where we could flatten by > >removing the cluster and letting the reference defer to the backing > >file? It's always safer to write 0 blocks into this image, but it may > >be worth considering whether we need the (ability) to try the alternate > >method as it results in a smaller file and potentially faster conversion. > This seems non-trivial to optimize to me (at least doing that check > fast), at least too non-trivial for implementing it solely for an > image version downgrade (which nobody who is concerned about image > size should do anyway, imho). > > For non-backed images however, we could certainly just unallocate > the blocks, I guess, since the spec explicitly states for that case > that "if a cluster is unallocated, read requests […] shall read > zeros for all parts that are not covered by the backing file" (also > applies if there is no backing file at all).
Yup, checking for !bs->backing_hd is easy, so simple deallocating in this case sounds like a good idea to do. Reading from the backing file and checking if the buffer is zero isn't _that_ complicated either, but at least the conversion speed won't be improved by doing this. If we already had Paolo'sbdrv_get_block_status, we could try that, but as it is today I don't think it's worth doing anything else here. Downgrading an image is an unusual operation anyway. Kevin
