On Wed, Aug 17, 2011 at 5:51 PM, Paolo Bonzini <pbonzini@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