Re: [Qemu-devel] [PATCH v2 00/17] blockdev-add QMP command

[Eric Blake]

On 10/01/2013 09:50 AM, Kevin Wolf wrote:
> Am 01.10.2013 um 17:41 hat Eric Blake geschrieben:
>> On 10/01/2013 07:20 AM, Kevin Wolf wrote:
>>> This series adds a blockdev-add QMP command and a row of cleanup patches to
>>> separate legacy -drive/drive_add behaviour from blockdev_init() to the 
>>> wrapper
>>> function drive_init().
>>>
>>> v2:
>>>
>>> - Removed I/O throttling and copy-on-read options from the schema: Both are
>>>   candidates for becoming block filters instead. For the time being, you can
>>>   use the existing QMP commands in order to enable I/O throttling on a 
>>> device
>>>   created with blockdev-add in a second step.
>>
>> Tolerable as a stop-gap; but doesn't that mean that there is a window
>> where throttling is not active?  Anything that can restrict block
>> operations must ultimately be specified atomically up front to avoid a
>> non-deterministic burst during the race window.  But in the interest of
>> incremental improvements, I can live with the approach used here, while
>> still waiting for throttling to be implemented as a true filter device.
> 
> Immediately after blockdev-add, the block device is still completely
> unused. You can set the throttling options before you do the
> corresponding device-add or block job command or whatever you're
> planning to do with the device.

Ah. So there's no race window after all.  Good to know.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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