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

[Eric Blake <eblake@redhat.com>]

On 9/26/20 8:32 AM, Vladimir Sementsov-Ogievskiy wrote:
> 25.09.2020 23:32, 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;
> 
> may be rename it to x-block-status ?

I thought about it.  We don't promise any back-compat with x- prefix, 
but at the same time, if there's not a strong reason to change it, 
scripts written against older qemu will not break as long as we still 
have the hack.

> 
>> 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
> 
> It wouldn't be so simple if we decide to export exact depth number..

It's still simple to do tri-state results if we separate the exact depth 
number to bits 31-4, leaving bits 1-0 as the tri-state summary...

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org