Re: [libvirt] [Qemu-devel] Killing block migration in qemu?
On Wed, Aug 17, 2011 at 5:51 PM, Paolo Bonzini <pbonzini(a)redhat.com> wrote:
following discussions yesterday with Juan Quintela and Marcelo
Tosatti, here
is my humble proposal: remove block migration from qemu master. It seems to
me that keeping block migration is going to slow down further improvements
on migration. The main problems are:
1) there are very good reasons to move migration to a separate thread. Only
a limited amount of extra locking, perhaps none is needed in order to do so
for RAM and devices. But the block drivers pretty much need to run under
the I/O thread lock, and coroutines will not help if the I/O thread is taken
by another thread. It's hard/unreliable/pointless to ping-pong migration
between threads.
The image streaming approach will also run in the I/O thread for the
mid-term future. Is the problem that the block migration code today
is too tied into the actual migration code path and therefore stops
from using it when migration happens in a separate thread?
2) there already are plans to reimplement block migration... it's called
streaming :) and not coincidentially it reuses some of the block migration
code.
What are the concrete issues with the existing block migration code?
I'm not disagreeing that we should move to a streaming approach but I
simply don't know the details of the existing block migration code.
Here is how it would go:
This sounds reasonable. In fact we can do both pre-copy and post-copy
block migration using streaming (+mirroring).
Stefan