[Qemu-devel] Image streaming gives live block copy for free (and vice versa)

[Stefan Hajnoczi]

Perhaps someone has been saying this all along but I want to spell it
out that image streaming and live block copy are equivalent in theory.
I just realized this last night.  In practice we might choose one
implementation or two different ones for performance reasons.

If any of these are unclear please let me know and I'll try to post diagrams.

Live block copy using image streaming
-------------------------------------

1. Create the destination image file and use the source image as the
backing file.
2. Quiesce I/O and pause VM.
3. Switch to destination image.
4. Resume VM.
5. Start streaming destination image in order to copy source image
data into destination file.
6. Streaming completes and disables the backing file, leaving the live
copied destination image that no longer depends on the source image.

There is no need for dirty block tracking because image streaming will
only copy over unallocated clusters.  There are no phases to the copy
process because the guest is writing to the destination file already
and does not dirty the source file.

Implementing live block copy without switch is also possible using the
block-mirror driver to update both the source image and the
destination image.  This would require making the backing file
writable though.

Image streaming using live block copy
-------------------------------------

1. Create destination image.
2. Start a live block copy to copy the source image data into destination file.
3. When live block copy reaches switch state, quiesce I/O and pause VM.
4. Switch to destination image, which now contains the flattened source image.
5. Resume VM.
7. Delete source image.

This approach copies the contents of the source image (and its backing
file) into the destination file.  You need to have two times the disk
space, since towards the end of live block copy you have two copies of
the image.  The QED image streaming patches I posted do not need twice
the disk space because they work in-place.

Call now!  We'll give you live snapshot merge using image streaming for FREE
----------------------------------------------------------------------------

1. Quiesce I/O and pause VM.
2. Free clusters in the snapshot that are allocated in the
copy-on-write delta file.
3. Make the copy-on-write delta file the backing file of the snapshot.
(Now the snapshot image has most clusters allocated except those which
were COWed due to a write after the snapshot was taken)
4. Resume VM.
5. Start streaming the snapshot image in order to copy the COW data
back into the snapshot file.
6. Streaming completes and disables the backing file, leaving the
merged snapshot.
7. Delete the COW file.

This approach is much more handwavy.  We needed to invert the backing
file relationship between snapshot and COW file, as well as freeing
clusters in order to make image streaming copy the data backing into
the snapshot file.

Stefan