Future qemu is expected to support these features (some already implemented):

Live features

Live block copy

  Ability to copy 1+ virtual disk from the source backing file/block 
  device to a new target that is accessible by the host. The copy 
  supposed to be executed while the VM runs in a transparent way.

Live snapshots and live snapshot merge

  Live snapshot is already incorporated (by Jes) in qemu (still need 
  virt-agent work to freeze the guest FS).
  Live snapshot merge is required in order of reducing the overhead 
  caused by the additional snapshots (sometimes over raw device).
  We'll use live copy to do the live merge

Image streaming (Copy on read)

  Ability to start guest execution while the parent image reside 
  remotely and each block access is replicated to a local copy (image 
  format snapshot)
  Such functionality can be hooked together with live block migration  
  instead of the 'post copy' method.

Live block migration (pre/post)

  Beyond live block copy we'll sometimes need to move both the storage 
  and the guest. There are two main approached here:
  - pre copy
    First live copy the image and only then live migration the VM.
    It is simple and safer approach in terms of management app, but if the 
    purpose of the whole live block migration was to balance the cpu load, it 
    won't be practical to use since copying an image of 100GB will take too long.
  - post copy (streaming / copy on read)
    First live migrate the VM, then on line stream its blocks.
    It's better approach for HA/load balancing but it might make 
    management complex (need to keep the source VM alive, handling failures)
  In addition there are two cases for the storage access:

  1. Shared storage
     Live block copy enable this capability, its seems like a rare case for live block migration.
  2.  There are some cases where the is no NFS/SAN storage and live migration is needed
     It should be similar to VMW's storage VM motion.
     http://www.vmware.com/files/pdf/VMware-Storage-VMotion-DS-EN.pdf
     http://www.vmware.com/products/storage-vmotion/features.html

Using external dirty block bitmap

  FVD has an option to use external dirty block bitmap file in 
  addition to the regular mapping/data files.
  We can consider using it for live block migration and live merge too.
  It can also allow additional usages of 3rd party tools to calculate  
  diffs between the snapshots.
  There is a big down side thought since it will make management 
  complicated and there is the risky of the image and its bitmap file 
  get out of sync. It's much better choice to have qemu-img tool to be 
  the single interface to the dirty block bitmap data.

Solutions

Non shared storage

  Either use iscsi (target and initiator) or NBD or proprietary qemu solution.
  iScsi in theory is the best but there is a problem of dealing with COW images -
  iScsi cannot report the COW level and detect un-allocated blocks. This might force us to use 
  proprietary solution.
  An interesting option (by Orit Wasserman) was to use iScsi for exporting the images 
  externally to qemu level and qemu will access as if they were a local device. This can  
  work well w/o almost any effort.  
  What do we do with chains of COW files? We create up to N such iscsi connections for every COW file in 
  the chain.
  [ Non shared storage using ISCSI]

Live block migration

  Use the streaming approach + regular live migration + iscsi:  
  Execute regular live migration and at the end of it, start streaming.
  If there is no shared storage, use the external iscsi and behave as if the image is 
  local. At the end of the streaming operation there will be a new local base image.

Block mirror layer

  Was invented in order to duplicate write IOs for the source and destination images.
  It prevents the potential race when both qemu and the management crash at the end of the
  block copy stage and it is unknown whether management should pick the source or the 
  destination

Streaming

  No need for mirror since only the destination changes and is writeable.

Block copy background task

  Can be shared between block copy and streaming

Live snapshot

  It can be seen as a (local) stream that preserve the current COW chain

Use cases

1. Basic streaming, single base master image on source storage, need to be instantiated on 
   destination storage

    The base image is a single level COW format (file or lvm).
    The base is RO and only new destination is RW. base' is empty at the beginning.
    The base image content is being copied in the background to base'. At the end of the 
    operation, base' is a standalone image w/o depending on the base image.

    a. Case of a shared storage streaming guest boot

    Before:           src storage: base             dst storage: none
    After             src storage: base             dst storage: base'

    b. Case of no shared storage streaming guest boot
       Every thing is the same, we use external iscsi target on the src host and external 
       iscsi initiator on the destination host.
       Qemu boots from the destination by using the iscsi access. This is transparent to 
       qemu (expect cmd syntax change ). Once the streaming is 
       over, we can live drop the usage of iscsi and open the image directly (some sort of 
       null live copy)

    c. Live block migration (using streaming) w/ shared storage.
       Exactly like 1.a. First create the destination image, then we run live migration 
       there w/o data in the new image. Now we stream like the boot scenario.

    d. Live block migration (using streaming) w/o shared storage.
       Like 1.b. + 1.c.

    *** There is complexity to handle multiple block device belonging to the same VM.
    *** Management will need to track each stream finish event and manage failures accordingly.

2. Basic streaming of raw files/devices

   Here we have an issue - what happens if there is a failure in the middle?
   Regular COW can sustain a failure since the intermediate base' contains information 
   dirty bit block information. Such a base' intermediate raw image will be broken. We  
   cannot revert back to the original base and start over because new writes were written 
   only to the base'.

   Approaches:
   a. Don't support that
   b. Use intermediate COW image and then live copy it into raw (waste time, IO, space)
      One can easily add new COW over the source and continue from there.
   c. Use external metadata of dirty-block-bitmap even for raw
   Suggestion: at this stage, do either recommendation #a or #b

3. Basic live copy, single base master image on source storage, need to be copied to the 
   destination storage

   The base image is a single level COW format or a raw file/device.
   The base image content is being copied in the background to base'. At the end of the 
   operation, base' is a standalone image w/o depending on the base image.
   In this case we only take into account a running VM, no need to do that for boot stage. 
   So it is either VM running locally and about to change its storage or
   a VM live migration.
   The plan is to use the mirror driver approach. Both src/dst are writable.

    a. Case of a shared storage, a VM changes its block device

    Before:           src storage: base             dst storage: none
    After             src storage: base             dst storage: base'

    This is a plain live copy w/o moving the VM.
    The case w/o shared storage seems not relevant here.
    We might want to move multiple block devices of the VM.
    It is written here for completeness - it shouldn't change anything.
    Still management/events will use the block name/id.

    b. Live block migration (w/o streaming) w/ shared storage.
       Unlike in the streaming case, the order here is reversed:
       Run live copy. When it ends and we're in the mirror state, run live migration.
       When it ends, stop the mirroring and make the VM continue on the destination.
       That's probably a rare use case.

    c. Live block migration (using streaming) w/o shared storage.
       Like 3.b. by using external iscsi

4. COW chains that preserve the full structure

   Before:           src storage: base <- sn1 <- snx       destination storage: none
   After             src storage: base <- sn1 <- snx       destination storage: base' <- sn1' <- snx'

    All of the original snapshot chains should be copied or stream as is to the new 
    storage. With copying we can do all of the non leaf images using standard 'cp tools'.
    If we're to use iscsi, we'll need to create N such connections.
    Probably not a common use case for streaming, we might ignore this and use this scenario only for 
    copying.

5. Like 4. but the chain can collapse. In fact this is like special case of #4
    
    Before:src storage: base <- sn1 <- sn2 .. <- snx  dst storage: none
    After: src storage: base <- sn1 <- sn2 ...<- snx  dst storage: base' <- sn1' .. <- sny'
   
   There is no difference from #4 other than collapsing some chain path into the dst leaf

6. Live snapshot

   It's here since the interface can be similar. Basically it is similar to live copy but 
   instead of copying, we switch to another COW on top. The only (separate) addition would 
   be to add a verb to ask the guest to flush its file systems.

   Before:           storage: base <- s1 <- sx        
   After             storage: base <- s1 <- sx <-sx+1

Exceptions

1. Hot unplug of the relevant disk
   Prevent that. (or cancel the operation)

1. Live migration in the middle of non migration action from above
   Shall we allow it? It can work but at the end of live migration we need to reopen the
   images (NFS mainly), it might add un-needed complexity.
   We better prevent that.

Interface

Image streaming API

Common implementation for live block copy and image streaming

Requirements

Live block copy:
- Open both source and destination read-write
- Mirror writes to source and destination
Image streaming:
- Source must be read-only
Be able to do a "partial" copy, i.e. share a backing file and don't copy data that can be read from this backing file
Use the COW provided by image formats, so that a crash doesn't force you to restart the whole copy
- Need a raw-cow driver that uses an in-memory COW bitmap for a raw image, so that we can use raw output images
- Anything that can do COW is good enough, so the implementation should be independent of the format
libvirt already has implemented block_stream QMP commands, so let's keep them unchanged if possible

Building blocks

Copy on read
- Improves time it takes to complete the copy
- Allows simpler implementation of copy
Background task doing the copy (by reading the whole image sequentially)
Mirroring of writes to source and destination
- source is a backing file of destination, i.e. we need to support writable backing files
Wrappers for live block copy and image streaming that enable the above features
Switching the backing file after the copy has completed (separate monitor command like in block copy?). Ends the mirroring if enabled.

Example (live block copy)

This section tries to explain by example what happens when you perform a live block copy. To make things interesting, we have a backing file (base) that is shared between source and destination, and two overlays (sn1 and sn2) that are only present on the source and must be copied.

So initially the backing file chain look like this:

 base <- sn1 <- sn2

We start by creating the copy as a new image on top of sn2:

 base <- sn1 <- sn2 <- copy

This gets all reads right automatically. For writes we use a mirror mechanism that redirects all writes to both sn2 and copy, so that sn2 and copy read the same at any time (and this mirror mechanism is really the only difference of live block copy from image streaming).

We start a background task that loops over all clusters. For each cluster, there are the following possible cases:

The cluster is already allocated in copy. Nothing to do.
Use something like bdrv_is_allocated() to follow the backing file chain. If the cluster is read from base (which is shared), nothing to do.
- For image streaming we're already on the destination. If we don't have shared storage and the protocol doesn't provide this bdrv_is_allocated() variant, we can still do something like qemu-img rebase and just compare the read data with data in base in order to decide if we need to copy.
Otherwise copy the cluster into copy

When the copy has completed, the backing file of copy is switched to base (in a qemu-img rebase -u way).

Features/LiveBlockMigration