On 9/26/20 2:33 AM, Richard W.M. Jones wrote:
On Fri, Sep 25, 2020 at 03:32:48PM -0500, Eric Blake wrote:
+The second is related to exposing the source of various extents within
+the image, with a single context named:
+
+ qemu:allocation-depth
+
+In the allocation depth context, bits 0 and 1 form a tri-state value:
+
+ bits 0-1 clear: NBD_STATE_DEPTH_UNALLOC, means the extent is unallocated
+ bit 0 set: NBD_STATE_DEPTH_LOCAL, the extent is allocated in this image
+ bit 1 set: NBD_STATE_DEPTH_BACKING, the extent is inherited from a
+ backing layer
From the cover description I imagined it would show the actual depth, ie:
top -> backing -> backing -> backing
depth: 1 2 3 .... (0 = unallocated)
I wonder if that is possible? (Perhaps there's something I don't
understand here.)
The real reason I don't want to do a straight depth number is that
'qemu-img map' combined with x-dirty-bitmap is still a very convenient
way to get at bits 0 and 1 (even if it requires decoding). But if we
plumb in a way for bdrv_get_status to return depth counts (rather than
reimplementing the depth count ourselves), I would have no problem with
returning a struct:
bits 31-4: the depth of the chain
bits 3-2: reserved (to make reading hex values easier...)
bits 1-0: tri-state of unalloc, local, or backing
where it would look like:
0x0000 -> unallocated
0x0011 -> depth 1, local
0x0022 -> depth 2, from the first backing layer
0x0032 -> depth 3, from the second backing layer
0x0042 ...
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org