Re: [PATCH for-8.2 v2 2/2] migration: Allow user to specify migration switchover bandwidth

[Wang, Lei]

On 9/6/2023 0:46, Peter Xu wrote:
> On Fri, Sep 01, 2023 at 09:37:32AM +0100, Daniel P. Berrangé wrote:
>>>> When the user wants to have migration only use 5Gbps out of that 10Gbps,
>>>> one can set max-bandwidth to 5Gbps, along with max-switchover-bandwidth to
>>>> 5Gbps so it'll never use over 5Gbps too (so the user can have the rest
>>>
>>> Hi Peter. I'm curious if we specify max-switchover-bandwidth to 5Gbps over a
>>> 10Gbps network, in the completion stage will it send the remaining data in 
>>> 5Gbps
>>> using downtime_limit time or in 10Gbps (saturate the network) using the
>>> downtime_limit / 2 time? Seems this parameter won't rate limit the final 
>>> stage:)
>>
>> Effectively the mgmt app is telling QEMU to assume that this
>> much bandwidth is available for use during switchover. If QEMU
>> determines that, given this available bandwidth, the remaining
>> data can be sent over the link within the downtime limit, it
>> will perform the switchover. When sending this sitchover data,
>> it will actually transmit the data at full line rate IIUC.
> 
> I'm right at reposting this patch, but then I found that the
> max-available-bandwidth is indeed confusing (as Lei's question shows).
> 
> We do have all the bandwidth throttling values in the pattern of
> max-*-bandwidth and this one will start to be the outlier that it won't
> really throttle the network.
> 
> If the old name "available-bandwidth" is too general, I'm now considering
> "avail-switchover-bandwidth" just to leave max- out of the name to
> differenciate, if some day we want to add a real throttle for switchover we
> can still have a sane name.
> 
> Any objections before I repost?

I'm also OK with it. "avail" has semantics that we have a lower bound of the
bandwidth when switchover so we can promise at least those amount of bandwidth
can be used, so it can cover both the throttling and non-throuttling case.
"switchover" means this parameter only works in the switchover phase rather than
the bulk stage.

> 
> Thanks,
>