Re: [PATCH 3/3] nbd: Add 'qemu-nbd -A' to expose allocation depth

[Richard W.M. Jones]

On Fri, Sep 25, 2020 at 03:32:49PM -0500, Eric Blake wrote:
> Allow the server to expose an additional metacontext to be requested
> by savvy clients.  qemu-nbd adds a new option -A to expose the
> qemu:allocation-depth metacontext through NBD_CMD_BLOCK_STATUS; this
> can also be set via QMP when using nbd-server-add.
> 
> qemu as client can be hacked into viewing this new context by using
> the now-misnamed x-dirty-bitmap option when creating an NBD blockdev;
> although it is worth noting the decoding of how such context
> information will appear in 'qemu-img map --output=json':
> 
> NBD_STATE_DEPTH_UNALLOC => "zero":false, "data":true
> NBD_STATE_DEPTH_LOCAL => "zero":false, "data":false
> NBD_STATE_DEPTH_BACKING => "zero":true, "data":true
> 
> libnbd as client is probably a nicer way to get at the information
> without having to decipher such hacks in qemu as client. ;)

I've been meaning to add extents information to nbdinfo, or
perhaps a new tool ("nbdmap").

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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