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

[Peter Xu <peterx@redhat.com>]

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?

Thanks,

-- 
Peter Xu