[Xen-devel] [RFC V8 3/3] xl snapshot-xxx Design

[Chunyan Liu <cyliu@xxxxxxxx>]

Changes to V7:
  * change wrong libxl_domain_snapshot_info naming to xl_domain_snapshot_info
  * remove all disk-only syntax
  * update xl snapshot-revert implementaion

===========================================================================
1. xl commandline interface design

xl snapshot-create:
  Create a snapshot (disk and RAM) of a domain.

  SYNOPSIS:
    snapshot-create <domain> [<cfgfile>] [--name <string>] [--live]

  OPTIONS:
    --name <string>  snapshot name
    --live           take a live snapshot

    If option includes --live, then the domain is not paused while creating
    the snapshot, like live migration. This increases size of the memory
    dump file, but reducess downtime of the guest.

    If option doens't include --name, a default name will be generated
    according to the creation time.

    If specify @cfgfile, cfgfile is prioritized. (e.g. if --name specifies
    a name, meanwhile there is name info in cfgfile, name in cfgfile will
    be used.)

xl snapshot-delete:
  Delete a snapshot (disk and RAM) of a domain.

  SYNOPSIS:
    snapshot-delete <domain> <snapshotname> [--children] [--children-only]

    By default, just this snapshot is deleted, and changes from this
    snapshot are automatically merged into children snapshots.

  OPTIONS:
    --children       delete snapshot and all children
    --children-only  delete children but not snapshot

    If option includes --children, then this snapshot and any descendant
    snapshots are deleted.

    If option include --children-only, only descendant snapshots are deleted,
    this snapshot is not deleted.

xl snapshot-revert:
  Revert domain to status of a snapshot.

  SYNOPSIS:
      snapshot-revert <domain> <snapshotname> [--running] [--paused] [--force]

  OPTIONS:
    --running        after reverting, change state to running
    --paused         after reverting, change state to paused
    --force          try harder on risky reverts

    Normally, the domain will revert to the same state the domain was in while
    the snapshot was taken (whether running, or paused).

    If option includes --running, then overrides the snapshot state to
    guarantee a running domain after the revert.

    If option includes --paused, then guarantees a paused domain after
    the revert.

xl snapshot-list:
  List snapshots for a domain.

  SYNOPSIS:
    snapshot-list <domain> [--parent] [--internal] [--external]
    [--tree] [--name]

  OPTIONS:
    --internal       filter by internal snapshots
    --external       filter by external snapshots
    --tree           list snapshots in a tree
    --parent         add a column showing parent snapshot
    --name           list snapshot names only

2. cfgfile syntax

"xl snapshot-create" supports creating a VM snapshot with user provided
configuration file. The configuration file syntax is as below:

#snapshot name. If user doesn't provide a VM snapshot name, xl will generate
#a name automatically by the creation time.
name=""

#snapshot description. Default is NULL.
description=""

#memory location. This field should be filled when memory=1. Default is NULL.
memory_path=""

#disk snapshot information
disks=['sda,1,qcow2,/tmp/sda_snapshot.qcow2','sdb,1,qcow2,/tmp/sda_snapshot.qcow2']
or
disks=['sda,0','sdb,0']

disk syntax:
'target device, external disk snapshot?, external format, external path'

3. xl structure to maintain VM snapshot info

xl_domain_snapshot_info = Struct("domain_snapshot_info",[
    # snapshot name
    ("name",          string)

    ("create_time",   string)
    ("description",   string)

    # memory path and disk snapshot info
    ("snapshot_args", libxl_domain_snapshot_args),

    # parent snapshot name
    ("parent",        string),

    # array to store all children snapshot name
    ("children",      Array(string, "num_children"),
    ]

According to xl_domain_snapshot_info, a json file will be saved on disk.

4. xl snapshot-xxx implementation details

"xl snapshot-create"

    1), parse args or domain snapshot configuration file.
    2), fill info in libxl_domain_snapshot_args struct according to
        options or config file.
    3), call libxl_domain_snapshot_create()
    4), fill info in xl_domain_snapshot_info.
    5), save snapshot info in json file under                                   
                          "/var/lib/xen/snapshots/domain_uuid"

"xl snapshot-list"

    1), read all domain snapshot related json file under
        "/var/lib/xen/snapshots/domain_uuid". Parse each file and fill in
        xl_domain_snapshot_info struct.
    2), display information from those xl_domain_snapshot_info(s)

"xl snapshot-delete"

    1), read snapshot json file from
        "/var/lib/xen/snapshots/domain_uuid/snapshotdata-<snapshot_name>\
        .libxl-json", parse the file and fill in xl_domain_snapshot_info
    2), according to parent/children info in xl_domain_snapshot_info
        and commandline options, decide which domain snapshot to be deleted.
        To delete each domain snapshot, fill in
        libxl_domain_snapshot_args and call libxl_domain_snapshot_delete().
    3), refresh parent/children relationship, delete json file for those
        already deleted snapshot.

"xl snapshot-revert"

    1), read snapshot json file from
        "/var/lib/xen/snapshots/domain_uuid/snapshotdata-<snapshot_name>\
        .libxl-json", parse the file and fill in xl_domain_snapshot_info.
    2), destroy current domain
    3). according to the info in xl_domain_snapshot_info, create a new
        domain from snapshot.

Interact with other operations:
All snapshots should be deleted before deleting a domain. This will
affact xl destroy/shutdown/save/migrate, adding related check and error
reporting.

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