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

[Alex Bligh]

On 4 Apr 2016, at 22:25, Eric Blake <address@hidden> wrote:

> On 04/04/2016 03:07 PM, Alex Bligh wrote:
>> 
>> On 4 Apr 2016, at 21:26, Eric Blake <address@hidden> wrote:
>> 
>>> 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.
>> 
>> OK, so you can ask for either (1) or (2), but not both. I see. I misread
>> that. I still think Denis's idea of selecting a bitmap to query works better.
> 
> I'm still not sure I follow what you are proposing in 'selecting a
> bitmap', at least not without also needing to expand the protocol to
> allow for a structured command that has more than a fixed-length message
> size (currently all commands except NBD_CMD_WRITE) or a self-described
> size (NBD_CMD_WRITE).

Whether it's an integer, or a UTF-8 string or whatever, we'd need
a way to expand the command structure.

An easy way to do that would be a command flag 'NBD_CMD_FLAG_EXTENDED_COMMAND'
which means the command was immediately followed by

0000 32-bit: command extra-length
0004 variable length data

>  Would this bitmap id be specified as an integer
> (32 bits?) as an arbitrary UTF-8 string? or as something else?

Or a 128 bit unique identifier (i.e. a packed UUID) to identify a bitmap.
That would obviate the need for a registry of such things.

That or the UTF-8 string

>  And
> since we _already_ documented that querying dirty information requires
> out-of-band coordination, why can't the out-of-band communication convey
> the bitmap selection, so that the NBD command then just reads the dirty
> status of the already-selected bitmap?

I suppose I was trying to make a single 'read bitmap' command, that
read an arbitrary bitmap. We then define one 'well known' bitmap
as 'allocation'. You can have your qemu bitmap(s) to do whatever
you want.

--
Alex Bligh

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