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

[Fam Zheng <famz@redhat.com>]

On Fri, 02/26 23:40, Denis V. Lunev wrote:
> On 02/26/2016 10:55 PM, Paolo Bonzini wrote:
> >
> >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
> >
> In general, the idea to bind DIRTY BITMAP to GET STATUS command is not that
> bad. First of all, NBD has no relation to the SCSI at all thus we are not
> bound to the SCSI protocol. This is good thing.
> 
> Next, it is generally good to query state of the data block before reading
> to reduce amount of transfers over the network. This is useful even for a
> full backup operation to avoid zero block transfers.
> 
> Thus, if we have created special NBD share, we can provide STATUS bitmap
> in the way we want, f.e. with the proposed semantics. This will not be a
> violation of the protocol.

Paolo described what I was trying to say, but way clearer then what I said. :)
And I agree a dedicated bitmap status seems better here.

Fam