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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
