Re: [PATCH 2/3] nbd: Add new qemu:allocation-depth metacontext

[Richard W.M. Jones]

On Mon, Sep 28, 2020 at 09:33:22AM -0500, Eric Blake wrote:
> 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 ...

This looks nice too.  However I was only bikeshedding so if any of
this is hard to do then don't worry too much.

Would like to add support for this map to nbdinfo too :-)

Rich.

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