Re: KVM call agenda for Apr 27

[Avi Kivity <avi@redhat.com>]

On 04/27/2010 12:08 PM, Dor Laor wrote:
> 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

There will be a large performance impact.

>>>
>>> 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.

I don't see how we can interleave messages from the plugin into the qmp 
stream without causing confusion.

-- 
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