Re: [Qemu-devel] [PATCH] migration: flush the bdrv before stopping VM

[Li, Liang Z]

> * Li, Liang Z (address@hidden) wrote:
> > > > > First explanation, why I think this don't fix the full problem.
> > > > > Whith this patch, we fix the problem where we have a dirty block
> > > > > layer but basically nothing dirtying the memory on the guest (we
> > > > > are moving the 20 seconds from max_downtime for the blocklayer
> > > > > flush), to 20 seconds until we have decided that the amount of
> > > > > dirty memory is small enough to be transferred during
> > > > > max_downtime.  But it is still going to take 20 seconds to flush
> > > > > the block layer, and during that 20 seconds, the amount of memory
> that can be dirty is HUGE.
> > > >
> > > > It's true.
> > >
> > > What kind of cache is it actually that takes 20s to flush here?
> > >
> >
> > I run a script in the guest which do a dd operation,  like this:
> >
> > #!/bin/sh
> > for i in {1..1000000}
> > do
> >     time dd if=/dev/zero of=/time.bdf bs=4k count=200000
> >     rm /time.bdf
> > done
> >
> > It's an extreme  case.
> 
> With what qemu options for the device, and what was your device backed by?

Very simple:
./qemu-system-x86_64 -enable-kvm -smp 4 -m 4096  -net none rhel6u5.img -monitor 
stdio

And it's a local migration.  I will do the test between two physical machines 
later.


Liang