Re: [PATCH v4 2/7] nbd: Add new qemu:allocation-depth metadata context

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

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 <eblake@redhat.com>

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

> ---
>   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