* Peter Xu ([email protected]) wrote: > Hi, > > I am playing with live migration and got one question about live > migration set_speed. > > Now we can use migrate_set_speed to configure the threshold during > migration (it should be only used for precopy, so let's assume the > migration is a precopy case). However I feel like this single > parameter cannot describe the process very clearly. > > The problem is, we use this "speed" value as both: > > 1. transfer threshold, to make sure migration stream is relatively > constant and under control > > 2. value to estimate "when we should stop the vm and start counting > downtime" > > For (1), we want a customized value that best suites our network > environment. E.g., if we have other critical network payloads, we can > set this to a very small number, so the migration stream will be very > small. > > For (2), it should be nothing else but possibly the network bandwidth > that the system can provide on the migration link.
Actually, that's what QEMU does; the migrate_set_speed is really only used for (1) directly. (2) actually comes from a measured bandwidth multiplied by the 'migrate_set_downtime' figure. (See around line 1800ish i migration/migration.c inside the if (current_time >= initial+time + BUFFER_DELAY) if ). > We can set_speed to a very small value to avoid disturbing other > network services, that's good. However using the same value to > estimate "when to stop" seems odd, since this value can be far away > from the real network speed (when we are transfering the last bits in > precopy, we should be using the max network speed all the time, which > actually is not affected by the threshold value). > > IMHO, it'll be clearer if these are two different tunables, e.g., not > sure whether it'll be cool to add another tunable "set_speed_max", to > configure the speed for the (2) case (when vm is stopped on source). > If so, it'll be clearer, and also bring another benefit: users can > have full control of the last migration phase as well, in case they > don't want to suffer from a network fluctuation for each ends of > migration. > > Still haven't thought further. Just throw this question out. I don't think people are very good at setting the two tunables we already have! Dave > > Thanks, > > -- peterx -- Dr. David Alan Gilbert / [email protected] / Manchester, UK
