Re: [Xen-devel] [RFC V9 4/4] domain snapshot design: libxl/libxlu

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Dec 16, 2014 at 02:32:57PM +0800, Chunyan Liu wrote:
> Changes to V8:
>   * remove libxl_domain_snapshot_create/delete/revert API
>   * export disk snapshot functionality for both xl and libvirt usage
> 
> ===========================================================================
> Libxl/libxlu Design
> 
> 1. New Structures
> 
> libxl_disk_snapshot = Struct("disk_snapshot",[
>     # target disk
>     ("disk",            libxl_device_disk),
> 
>     # disk snapshot name
>     ("name",            string),
> 
>     # internal/external disk snapshot?
>     ("external",        bool),
> 
>     # for external disk snapshot, specify following two field
>     ("external_format", string),
>     ("external_path",   string),
>     ])
> 

So you don't propose making libxl to have knowledge of the snapshot
chains? And in libvirt (or other toolstack that's interested in snapshot
management) you represent a snapshot as chains (or trees) of
libxl_disk_snapshot? (Not suggesting you do things the other way around,
just to confirm)

> 
> 2. New Functions
> 
> Since there're already APIs for saving memory (libxl_domain_suspend)
> and restoring domain from saved memory (libxl_domain_create_restore), to
> xl domain snapshot tasks, the missing part is disk snapshot functionality.
> And the disk snapshot functionality would be used by libvirt too.
> 
> Considering there is qmp handling in creating/deleting disk snapshot,
> will add following new functions to libxl (?):
> 
> int libxl_disk_snapshot_create(libxl_ctx *ctx, uint32_t domid,
>                                libxl_disk_snapshot *snapshot, int nb);
> 
>     Taking disk snapshots to a group of domain disks according to
>     configuration. For qcow2 disk backend type, it will call qmp
>     "transaction" command to do the work. For other disk backend types,
>     might call other external commands.
> 
>     Parameters:
>        ctx (INPUT):
>            context
>        domid (INPUT):
>            domain id
>        snapshot (INPUT):
>            array of disk snapshot configuration. Has "nb" members.
> 
>            libxl_device_disk:
>                structure to represent which disk.
>            name:
>                snapshot name.
>            external:
>                internal snapshot or external snapshot.
>                'false' means internal disk snapshot. external_format and
>                external_path will be ignored.
>                'true' means external disk snapshot, then external_format
>                and external_path should be provided.
>            external_format:
>                Should be provided when 'external' is true. If not provided,
>                will use default format proper to the backend file.
>                Ignored when 'external' is false.
>            external_path:
>                Must be provided when 'external' is true.
>                Ignored when 'external' is false.
>        nb (INPUT):
>            number of disks that need to take disk snapshot.
> 
>     Return:
>        0 on success, -1 on error.
> 

It should return appropriate libxl error code (ERROR_*) on error.

> 
> /*  This API might not be used by xl, since xl won't take care of deleting
>  *  snapshots. But for libvirt, since libvirt manages snapshots and will
>  *  delete snapshot, this API will be used.
>  */
> int libxl_disk_snapshot_delete(libxl_ctx *ctx, uint32_t domid,
>                                libxl_disk_snapshot *snapshot, int nb);
> 
>     Delete disk snapshot of a group of domain disks according to
>     configuration. For qcow2 disk backend type, it will call qmp command
>     to delete internal disk snapshot. For other disk backend types, might
>     call other external commands.
> 
>     Parameters:
>        ctx (INPUT):
>            context
>        domid (INPUT):
>            domain id
>        snapshot (INPUT):
>            array of disk snapshot configuration. Has "nb" members.
>        nb (INPUT):
>            number of disks that need to take disk snapshot.
> 
>     Return:
>        0 on success, -1 on error.
> 
> 
> int libxl_disk_to_snapshot(libxl_ctx *ctx, uint32_t domid,
>                            libxl_disk_snapshot **snapshot, int *num);
> 
>     This is for domain snapshot create. If user doesn't specify disks,
>     then by default it will take internal disk snapshot to each domain
>     disk. This function will fill libxl_disk_snapshot according to domain
>     disks info.
> 
>     Parameters:
>        ctx (INPUT):
>            context
>        domid (INPUT):
>            domain id
>        snapshot (OUTPUT):
>            array of disk snapshot configuration.
>        num (OUTPUT):
>            number of disks.
> 
>     Return:
>        0 on success, -1 on error.
> 
> 
> 
> For disk snapshot revert, no qmp command for that, it always calls
> external commands to finish the work, so put in libxlu (?):
> 
> int xlu_disk_snapshot_revert(libxl_disk_snapshot *snapshot, int nb);
> 

IMHO it's fine for this to be in libxl. Calling out to other programs
is fine.

Wei.

>     Apply disk snapshot for a group of disks according to configuration. To
>     different disk backend types, call different external commands to do
>     the work.
> 
>     Parameters:
>        snapshot (INPUT):
>            array of disk snapshot configuration. Has "nb" members.
>        nb (INPUT):
>            number of disks that need to take disk snapshot.
> 
>     Return:
>        0 on success, -1 on error.
> 
> 
> 
> 3. Simple Architecture View
> 
> Creating domain snapshot:
> (* means new functions we will need in libxl/libxlu)
> 
>   "xl snapshot-create"
>          |
>   parse configuration ----> libxl_disk_to_snapshot (*)
>          |
>   saving memory ----> libxl_domain_suspend
>          |
>  taking disk snapshot ----> libxl_disk_snapshot_create (*)
>          |                     |
>          |                     --> libxl_qmp_disk_snapshot_transaction (*)
>          |
>     unpause domain ---->libxl_domain_unpause
>          |
>         End
> 
> 
> Reverting to a snapshot:
> (* means new functions we will need in libxl/libxlu)
> 
>   "xl snapshot-revert"
>          |
>    parse configuration
>          |
>    destroy domain ---->libxl_domain_destroy
>          |
>  reverting disk snapshot ----> xlu_disk_snapshot_revert (*)
>          |                       |
>          |                       --> call 'qemu-img' to apply disk snapshot
>          |
>  restore domain from saved memory ----> libxl_domain_create_restore
>          |
>         End

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