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

[Dr. David Alan Gilbert]

* 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?

Dave

--
Dr. David Alan Gilbert / address@hidden / Manchester, UK