10.10.2020 00:55, Eric Blake wrote:
'qemu-img map' provides a way to determine which extents of an image
come from the top layer vs. inherited from a backing chain. This is
useful information worth exposing over NBD. There is a proposal to
add a QMP command block-dirty-bitmap-populate which can create a dirty
bitmap that reflects allocation information, at which point the
qemu:dirty-bitmap:NAME metadata context can expose that information
via the creation of a temporary bitmap, but we can shorten the effort
by adding a new qemu:allocation-depth metadata context that does the
same thing without an intermediate bitmap (this patch does not
eliminate the need for that proposal, as it will have other uses as
well).
For this patch, I just encoded a tri-state value (unallocated, from
this layer, from any of the backing layers); an obvious extension
would be to provide the actual depth in bits 31-4 while keeping bits
1-0 as a tri-state (leaving bits 3-2 unused, for ease of reading depth
from a hex number). But adding this extension would require
bdrv_is_allocated_above to return a depth number.
While documenting things, remember that although the NBD protocol has
NBD_OPT_SET_META_CONTEXT, the rest of its documentation refers to
'metadata context', which is a more apt description of what is
actually being used by NBD_CMD_BLOCK_STATUS: the user is requesting
metadata by passing one or more context names. So I also touched up
some existing wording to prefer the term 'metadata context' where it
makes sense.
Note that this patch does not actually enable any way to request a
server to enable this context; that will come in the next patch.
Signed-off-by: Eric Blake
Reviewed-by: Vladimir Sementsov-Ogievskiy
---
docs/interop/nbd.txt | 27 ++---
[..]
+In the allocation depth context, bits 0 and 1 form a tri-state value:
+
+bits 0-1: 00: NBD_STATE_DEPTH_UNALLOC, the extent is unallocated
+ 01: NBD_STATE_DEPTH_LOCAL, the extent is allocated in the
+ top level of the image
Hmm. I always thought that "image" == file, so backing chain is a chain of
images,
not a several levels of one image. If it is so, than it should be "the top level
image".
And "levels of the image" may designate internal qcow2 snapshots unrelated
here..
--
Best regards,
Vladimir
