Re: [Qemu-devel] Image streaming and live block copy

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

On Thu, Jun 16, 2011 at 11:52:43AM -0300, Marcelo Tosatti wrote:
This approach does not use the backing file feature?

> blkstream block driver:
> 
> - Maintain in memory whether given block is allocated in local image,
> if not, read from remote, write to local. Set block as local.
> Local and remote simply two block drivers from image streaming driver
> POV.
> - Once all blocks are local, notify mgmt so it can switch to local
> copy.
> - Writes are mirrored to source and destination, minding guest writes
> over copy writes.

We open the remote file read-only for image streaming and do not want to
mirror writes.

If QEMU crashes or there is a power failure we need to restart the
streaming process carefully - local blocks must not be overwritten.
Perhaps this is the tricky part.

> Over this scheme, you'd have:
> 
> 1) Block copy. 
> Reopen image to be copied with
> blkstream:/path/to/current-image:/path/to/destination-image,
> background read sectors 0...N.
> 
> 2) Image stream:
> blkstream:remote-image:/path/to/local-image,
> background read sectors 0...N.

Stefan