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

[Daniel P . Berrangé]

On Tue, Sep 05, 2023 at 12:46:03PM -0400, 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 think the 'avail-' prefix is good given the confusion Lei pointed out.

With regards,
Daniel
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