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

[Li, Liang Z]

> > > 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.