[Xen-devel] [RFC V9 2/4] domain snapshot overview

[Chunyan Liu <cyliu@xxxxxxxx>]

Changes to V8:
  * add an overview document, so that one can has a overall look
    about the whole domain snapshot work, limits, requirements,
    how to do, etc.

=====================================================================
Domain snapshot overview

1. Purpose

Domain snapshot is a system checkpoint of a domain. Later, one can
roll back the domain to that checkpoint. It's a very useful backup
function. A domain snapshot contains the memory status at the
checkpoint and the disk status (which we called disk snapshot).

Domain snapshot functionality usually includes:
a) create a domain snapshot
b) roll back (or called "revert") to a domain snapshot
c) delete a domain snapshot
d) list all domain snapshots

But following the existing xl idioms of managing storage and saved
VM images via existing CLI command (qemu-img, lvcreate, ls, mv,
cp etc), xl snapshot functionality would be kept as simple as
possible:
* xl will do a) and b), creating a snapshot and reverting a
  domain to a snapshot.
* xl will NOT do c) and d), xl won't manage snapshots, as xl
  doesn't maintain saved images created by 'xl save'. So xl
  will have no idea of the existence of domain snapshots and
  the chain relationship between snapshots. It will depends on
  user to take care of the snapshots, know the snapshot chain
  info, and delete snapshots.

Domain Snapshot Support and Not Support:
* support live snapshot
* support internal disk snapshot and external disk snapshot
* support different disk backend types.
  (Basic goal is to support 'raw' and 'qcow2' only).

* not support snapshot when domain is shutdowning or dying.
* not support disk-only snapshot [1].

 [1] To xl, it only concerns active domains, and even when domain
 is paused, there is no data flush to disk operation. So, take
 a disk-only snapshot and then resume, it is as if the guest
 had crashed. For this reason, disk-only snapshot is meaningless
 to xl. Should not support.


2. Requirements

General Requirements:
* ability to save/restore domain memory
* ability to create/delete/apply disk snapshot [2]
* ability to parse user config file

  [2] Disk snapshot requirements:
  - external tools: qemu-img, lvcreate, vhd-util, etc.
  - for basic goal, we support 'raw' and 'qcow2' backend types
    only. Then it requires:
    libxl qmp command or "qemu-img" (when qemu process does not
    exist)


3. Interaction with other operations:

No.


4. General workflow

Create a snapshot:
  * parse user cfg file if passed in
  * check snapshot operation is allowed or not
  * save domain, saving memory status to file (refer to: save_domain)
  * take disk snapshot (e.g. call qmp command)
  * unpause domain

Revert to snapshot:
  * parse use cfg file (xl doesn't manage snapshots, so it has no
    idea of snapshot existence. User MUST supply configuration file)
  * destroy this domain
  * create a new domain from snapshot info
    - apply disk snapshot (e.g. call qemu-img)
    - a process like restore domain

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