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).
+
+ /* the refcount order might be different in newer images - however, qemu
+ * doesn't support anything different than 4 anyway, so nothing to fix
+ * there */
This sounds risky. Wouldn't it be safer to error out if the image
didn't have a refcount order of 4, than to just ignore it; on the
grounds that if qemu DOES add support for non-4 refcount order, an error
will at least alert someone to the fact that they need to add some
(potentially complicated) code here?
Oh, yes, of course. I'll fix it.
Max
