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

From: Denis V. Lunev
Subject: Re: [Qemu-devel] [Nbd] [PATCH v2] doc: Add NBD_CMD_BLOCK_STATUS extension
Date: Mon, 4 Apr 2016 23:04:19 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1

On 04/04/2016 10:58 PM, Alex Bligh wrote:

Nothing requires the two uses to report at the same granularity.  THe
NBD_REPLY_TYPE_BLOCK_STATUS allows the server to divide into descriptors
as it sees fit (so it could report holes at a 4k granularity, but
dirtiness only at a 64k granularity) - all that matters is that when all
the descriptors have been sent, they total up to the length of the
original client request.  So by itself, granularity does not require
another command.
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.

At this point, I was just trying to rebase the proposal as originally
made by Denis and Pavel; perhaps they will have more insight on how they
envisioned using the command, or on whether we should try harder to make
this more of a two-way protocol (where the client can tell the server
when to mark something as clean, or when to start tracking whether
something is dirty).
I'm not sure it needs to be a two way protocol (see my strawman) but
I'd like to know it's at least possibly useful.

Alex Bligh

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