Re: [Qemu-block] [PATCH v5 17/38] block: Add BlockBackendRootState

[Eric Blake]

On 09/29/2015 05:17 AM, Alberto Garcia wrote:
> On Tue 22 Sep 2015 05:00:05 PM CEST, Max Reitz wrote:
> 
>>> The correct way to solve this seems to be that each BB has its own
>>> I/O throttling filter. Actually, if we could lift the throttling code
>>> to BlockBackend, that might solve the problem.
>>
>> So yes, as long as we have throttling on the BDS level, something may
>> always break when having multiple BBs per BDS, because you'd probably
>> want throttling to be on the BB level. But lifting that doesn't seem a
>> trivial task, and I don't really know whether I want this in this
>> series.
>>
>> Especially considering that right now you cannot have multiple BBs per
>> BDS.
>>
>> All in all: Yes, before we allow multiple BBs per BDS we want
>> throttling to be on the BB level. But this is nothing that should be
>> done in or for this series, since right now we do not allow multiple
>> BBs per BDS.
> 
> I agree that it makes sense to move throttling to the BB level. It's
> probably not trivial but I don't think it's too complex either. I can
> give it a look once this series has been merged.

Ultimately, I think we want throttling at both levels.  I can make the
argument that in a cloud, a guest should not be able to consume more
than X resources (throttling at the BB, regardless of what BDS tree it
is associated with); but I can also argue that I may want to throttle
specific BDS (a backing chain with network backing file and local active
file: throttle the backing file to limit network traffic, and the guest
gets faster as it moves more local; or conversely, a common backing file
and one-off guest: throttle the active layer to do performance analysis
of the bottlenecks where the guest differs from the common backing layer).

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

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