Re: [Qemu-devel] [PATCH v2 0/6] external backup api

[Paolo Bonzini]

On 19/02/2016 09:51, Markus Armbruster wrote:
>> Is it an abuse to "Get LBA Status" to return dirty information? Because in 
>> SCSI
>> the command reports "mapped", "allocated" and "anchored" statuses. Does that
>> mean NBD will use a different status set?
> 
> Perhaps some conceptual gymnastics can get us to standard semantics.
> 
> Incremental backup wants to copy out an image's "dirty" blocks.
> 
> We can view this as a bitmap telling us which of the image's blocks are
> dirty.
> 
> An alternative view would be base image + dirty delta image.  In the the
> dirty delta, exactly the dirty blocks are allocated.  The delta image
> may be conceptual.

I see a problem here. On one hand I agree that the "GET LBA STATUS" is
a natural extension to the NBD protocol.

On the other hand, the Get LBA Status command in SCSI reflects the
state over the whole chain, not only the top element.  It is the
equivalent of bdrv_get_block_status_above(bs, NULL, ...), rather than
bdrv_get_block_status(bs, ...).  My understanding is that the dirty
block information would require the latter, especially in the
"conceptual delta image" model that Markus describes above.

Having NBD implement subtly different semantics compared to SCSI is a
bad idea in my opinion.

Of course if we call it "READ DIRTY BLOCKS" that would work, but I
don't think such a command would be something that it makes sense to
have in the general purpose NBD spec, so you would need a mechanism
for vendor-specific extensions.

Paolo

> Now, consider the dirty delta *without* its backing image.  You can find
> its allocated blocks with Get LBA Status.  As usual, you can read even
> unallocated blocks, see SBC3 table 9.
> 
> If we NBD-export exactly that, we can use standard semantics, can't we?