On Tue, Mar 04, 2014 at 01:02:44AM +0000, Huangpeng (Peter) wrote: > > But back to the options: > > > > If the host has enough free memory to fork QEMU, a small helper process can > > be used to save the copy-on-write memory snapshot (thanks to fork(2) > > semantics). The hard part about the fork(2) approach is that QEMU isn't > > really designed to fork, so work is necessary to reach a quiescent state > > for the > > child process. > > > > If there is not enough memory to fork, then a synchronous approach to > > catching guest memory writes is needed. I'm not sure if a good mechanism > > for that exists but the simplest would be mprotect(2) and a signal handler > > (which will make the guest run very slowly). > > > > Stefan > > In real production environment, memory over-commit or use as much memory as > possible may be the normal case, so the fork semantics cannot meet the needs. >
Yes, I think you're right. The fork approach only works in the easy case where there is plenty of free host memory. > Is there any other proposals to implement vm-snapshot? See the discussion by Paolo and Andrea about post-copy migration, which adds kernel memory management features for tracking userspace page faults. Perhaps you can use that infrastructure to trap guest writes. Stefan
