[Qemu-devel] Question with migrate_set_speed

[Peter Xu]

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.

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.

Thanks,

-- peterx