Re: [Qemu-discuss] [Qemu-devel] Live Block Migration Strategy

[Stefan Hajnoczi]

On Fri, Apr 19, 2013 at 6:11 AM, Yaodong Yang <address@hidden> wrote:
> Could someone share me with some information about the live block migration
> strategy in the current qemu-kvm. As I know, there are at least IO
> mirroring, dirty block tracking, snapshot, precopy or postcopy strategy
> exist for the block migration.

migrate -b is the traditional "block migration" feature in qemu-kvm.
It copies the contents of a block device as part of the live migration
stream.  This is a pre-copy approach using a dirty bitmap.  It is
being replaced by more flexible alternatives.

block-stream can be used for pre- and post-copy migration.  It creates
a new image file that uses the old disk image as its backing file.
The contents of the backing file are copied into the new image unless
new blocks have already been written in the new image.  It requires
NFS, NBD, or another mechanism to access both the new image and the
backing file at the same time.  It is not part of the live migration
stream.

drive-mirror is even more flexible; one way to use it is to pre-copy
the contents of the disk to the destination host.  It does not use
backing files to achieve this, so the destination image can be any
image format.  Again, it requires NFS, NBD, or another mechanism to
access both the new image and the old image.  It is not part of the
live migration stream.  An advantage of the mirroring approach over
block-stream is that you can continue running the old image if there
is a power failure or crash during mirroring - simply discard the new
image and start over.

Stefan