Re: [PATCH RFC 2/2] migration: abort on destination if switchover limit exceeded

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Jun 26, 2024 at 12:29:41PM +0100, Joao Martins wrote:
> On 25/06/2024 19:37, Daniel P. Berrangé wrote:
> > On Tue, Jun 25, 2024 at 10:53:41AM -0400, Peter Xu wrote:
> >> Then the question is how should we suggest the user to specify these two
> >> parameters.
> >>
> >> The cover letter used:
> >>
> >>   migrate_set_parameter downtime-limit 300
> >>   migrate_set_parameter switchover-limit 10
> > 
> > What this means is that in practice the total downtime limit
> > is 310 ms, however, expressing this as two parameters is
> > incredibly inflexible.
> > 
> > If the actual RAM transfer downtime only took 50 ms, then why
> > should the switchover downtime still be limited to 10ms, when
> > we've still got a budget of 250 ms that was unused.
> > 
> 
> The downtime limit is 300, it's more than you are giving something *extra* 10ms
> when you switchover regardless of where that's spent.
> 
> If it makes it easier to understand you could see this parameter as:
> 
> 'downtime-limit-max-error' = 10 ms
> 
> The name as proposed by the RFC was meant to honor what the error margin was
> meant for: to account for extra time during switchover. Adding this inside
> downtime-limit wouldn't work as it otherwise would be used solely for RAM
> transfer during precopy.
> 
> > IOW, if my VM tolerates a downtime of 310ms, then I want that
> > 310ms spread across the RAM transfer downtime and switchover
> > downtime in *any* ratio. ALl that matters is the overall
> > completion time.
> > 
> That still happens with this patches, no specific budget is given to each.

If no specific budget is given to each, then IMHO adding the second
parameter is pointless & misleading. 

With regards,
Daniel
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