nbd: Add 'qemu-nbd -A' to expose allocation depth

Allow the server to expose an additional metacontext to be requested
by savvy clients.  qemu-nbd adds a new option -A to expose the
qemu:allocation-depth metacontext through NBD_CMD_BLOCK_STATUS; this
can also be set via QMP when using block-export-add.

qemu as client is hacked into viewing the key aspects of this new
context by abusing the already-experimental x-dirty-bitmap option to
collapse all depths greater than 2, which results in a tri-state value
visible in the output of 'qemu-img map --output=json' (yes, that means
x-dirty-bitmap is now a bit of a misnomer, but I didn't feel like
renaming it as it would introduce a needless break of back-compat,
even though we make no compat guarantees with x- members):

unallocated (depth 0) => "zero":false, "data":true
local (depth 1)       => "zero":false, "data":false
backing (depth 2+)    => "zero":true,  "data":true

libnbd as client is probably a nicer way to get at the information
without having to decipher such hacks in qemu as client. ;)

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20201027050556.269064-11-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/docs/tools/qemu-nbd.rst b/docs/tools/qemu-nbd.rst
index 667861cb22..fe41336dc5 100644
--- a/docs/tools/qemu-nbd.rst
+++ b/docs/tools/qemu-nbd.rst
@@ -72,10 +72,16 @@ driver options if ``--image-opts`` is specified.
 
   Export the disk as read-only.
 
+.. option:: -A, --allocation-depth
+
+  Expose allocation depth information via the
+  ``qemu:allocation-depth`` metadata context accessible through
+  NBD_OPT_SET_META_CONTEXT.
+
 .. option:: -B, --bitmap=NAME
 
   If *filename* has a qcow2 persistent bitmap *NAME*, expose
-  that bitmap via the ``qemu:dirty-bitmap:NAME`` context
+  that bitmap via the ``qemu:dirty-bitmap:NAME`` metadata context
   accessible through NBD_OPT_SET_META_CONTEXT.
 
 .. option:: -s, --snapshot