[Qemu-devel] Re: KVM call agenda for Apr 27

[Dor Laor <dlaor@redhat.com>]

On 04/27/2010 11:56 AM, Avi Kivity wrote:
> On 04/27/2010 11:48 AM, Dor Laor wrote:
>>> Here's another option: an nbd-like protocol that remotes all BlockDriver
>>> operations except read and write over a unix domain socket. The open
>>> operation returns an fd (SCM_RIGHTS strikes again) that is used for read
>>> and write. This can be used to implement snapshots over LVM, for
>>> example.
>>>
>>
>>
>> Why w/o read/writes?
>
> To avoid the copying.

Of course, just pass the offset+len on read/write too

>
>> the watermark code needs them too (as info, not the actual buffer).
>
> Yeah. It works for lvm snapshots, not for watermarks.
>
>>
>> IMHO the whole thing is way over engineered:
>> a) Having another channel into qemu is complicating management
>> software. Isn't the monitor should be the channel? Otherwise we'll
>> need to create another QMP (or nbd like Avi suggest) for these
>> actions. It's extra work for mgmt and they will have hard time to
>> understand events interleaving of the various channels
>
> block layer plugins allow intercepting all interesting block layer
> events, not just write-past-a-watermark, and allow actions based on
> those events. It's a more general solution.

No problem there, as long as we do try to use the single existing QMP 
with the plugins. Otherwise we'll create QMP2 for the block events in a 
year from now.

>
>> b) How the plugins are defined? Is it scripts? Binaries? Do they open
>> their own sockets?
>
> Shared objects.
>
>