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Re: [Qemu-devel] [Nbd] [PATCH v2] doc: Add NBD_CMD_BLOCK_STATUS extensio

From: Eric Blake
Subject: Re: [Qemu-devel] [Nbd] [PATCH v2] doc: Add NBD_CMD_BLOCK_STATUS extension
Date: Mon, 4 Apr 2016 14:26:16 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1

On 04/04/2016 02:08 PM, Alex Bligh wrote:
> On 4 Apr 2016, at 21:04, Denis V. Lunev <address@hidden> wrote:
>>> Sure, but given you can't report dirtiness without also reporting
>>> allocation, if they are are at different blocksize I'd rather they
>>> were in different commands, because otherwise the code to report
>>> block size needs to work at two different granularities.
>> 'dirty' could come after the data was 'trimmed' from the region!
>> thus we could have dirty unallocated data.
> Let me rephrase.
> Under the current proposal it is not possible to report whether or
> not a region is dirty without also reporting whether or not it
> is allocated.

Huh? The current proposal _requires_ these to be separate queries.  You
cannot query dirtiness at the same time as allocation, because the value
of NBD_CMD_FLAG_DIRTY is distinct between the two queries.

> As these two concepts exist at potentially
> different block sizes, the code to support reporting on allocation
> must now be able to run both at the allocation blocksize and
> the dirtiness blocksize, which is going to be a pain.

No, the code for reporting allocation does NOT have to be run at the
same time as when reporting dirtiness.

> If these were two different commands, they could each run at their
> natural block size.

Both modes of operation already can run at their natural block size.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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