From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jim Fehlig <jfehlig@suse.com>
Subject: Re: [RFC v2] vm snapshot documents
Date: Tue, 22 Apr 2014 21:31:19 -0600
Message-ID: <53573407.3090609@suse.com>
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To: Bamvor Jian Zhang <bjzhang@suse.com>
Cc: Zhiqiang Zhou <ZZhou@suse.com>, Ian Campbell <Ian.Campbell@citrix.com>, Ian Jackson <Ian.Jackson@eu.citrix.com>, Chun Yan Liu <CYLiu@suse.com>, xen-devel@lists.xen.org, Anthony.perard@citrix.com
List-Id: xen-devel@lists.xenproject.org

Bamvor Jian Zhang wrote:
> Hi,
>
> here is the second version about vm snapshot documents, the first version is
> here[1]
>   

Hi Bamvor,

Thanks for being adventurous and starting this work :-).

> thanks Ian Jackson suggestion, i write this doc about vm snapshot including new
> command, struct and api. currently, the disk snapshot(create, delete, list)
> worked(patch is already sent[2]), and vm snapshot(create, delete, list) work.
> but i have some questions about snapshot apply(see the last section).
>
> feel free to comment it. thanks.
>
> 1, new command
> =head1 SNAPSHOT
>
> there are two types of snapshots supported by libxl: disk snapshot and vm
> snapshot.

Here and throughout, you should be consistent with the use of 'vm'
verses 'domain'. For the most part, the existing docs use 'domain'.

It would be good to provide more detail about these types of snapshots
too. E.g. a disk snapshot will only be crash-consistent if the domain is
running. Disk snapshots can also be internal (qcow2) or external
(snapshot in one file, delta in another).

Domain snapshots include disk snapshots and domain state, allowing to
resume the domain from the same state when the snapshot was created.
This type of snapshot is also referred to as a domain checkpoint or
system checkpoint.

>  The following subcommands management the snapshot of a domain,
> including create, delete, list and apply.
>   

I struggle over apply vs revert, but lean toward the latter.

> a domain snapshot(or a vm snapshot) means save the domain config, memory and
> disk snapshot. currently, only support qcow2 and internal disk snapshot.
>
> the vm snapshot information will store in the follow path:
> /var/lib/xen/snapshots/<domain_uuid>/snapshotdata-<snapshot_name>.xl
>
> here is an example for snapshot information file:
> snapshot_name="1397207577"
> snapshot_creationtime="1397207577"
> snapshot_save="/var/lib/xen/snapshots/5c84adcc-bd59-788a-96d2-195f9b599cfe/1397207577.save"
>   

The snapshot prefix seems redundant. A 'description' entry would be
useful too.

> the user could give a snapshot name when vm snapshot created. if not, the epoch
> seconds will set as name as the above examples.
>
> =over 4
>
> =item B<vm-snapshot-create> [I<OPTIONS>] I<domain-id>
>
> create vm snapshot.
> it will call the qmp transaction for creating the disk snapshot in order to
> ensure the disk snapshot is coherence.
> vm is paused during snapshot create, and is unpause after take snapshot
> finished.
>   

I think the user should be able to choose whether to live snapshot vs
pausing/unpausing, via an option. E.g. 'xl snapshot-create <dom> ...
--live'.

> B<OPTIONS>
>
> =over 4
>
> =item B<-n>
>
> vm snapshot name
>
> =back
>
> =item B<vm-snapshot-delete> [I<OPTIONS>] I<domain-id>
>   

This should take a snapshot name, since a domain could have several.

> delete vm snapshot.
> delete the saved memory file and snapshot information file created by
> vm-snapshot-create.
>
> B<OPTIONS>
>
> =over 4
>
> =item B<-n>
>
> vm snapshot name
>
> =back
>
> =item B<vm-snapshot-list> I<domain-id>
>
> list vm snapshot for the dedicated domain including snapshot name and creation
> time which stored in the vm snapshot information file.
>   

This could behave like domain listing in xl. E.g. 'xl snapshot-list
<dom>' will list all snapshots. 'xl snapshot-list --long <dom>' will
list detailed info about all snapshots. 'xl snapshot-list --long <dom>
<snapshot-name>' will list detailed info on the requested snapshot.

> =item B<vm-snapshot-apply> [I<OPTIONS>] I<domain-id>
>
> apply vm snapshot for the dedicated domain.
>
> B<OPTIONS>
>
> =over 4
>
> =item B<-n>
>
> vm snapshot name
>
> =back
>
> =item B<disk-snapshot-create> [I<OPTIONS>] I<domain-id>
>
> create disk snapshot.
>
> B<OPTIONS>
>
> =over 4
>
> =item B<-n>
>
> disk snapshot name
>
> =back
>
> =item B<disk-snapshot-delete> [I<OPTIONS>] I<domain-id>
>
> delete disk snapshot by snapshot name or snapshot id.
>
> B<OPTIONS>
>
> =over 4
>
> =item B<-i>
>
> disk snapshot id
>
> =item B<-n>
>
> disk snapshot name
>
> =back
>
> =item B<disk-snapshot-list> I<domain-id>
>
> list disk snapshot including snapshot id, tag, vm size, date, vm clock.
>
> =back
>   

IMO, the vm-snapshot-* and disk-snapshot-* subcommands can be combined
into one snapshot-*, and passing an option for disk only snapshots. E.g.
'xl snapshot-create <dom> ... --disk-only'.

> 2, new struct and new api
> 1), new struct
> (1), libxl_snapshot struct store a disk snapshot information, which get from
> qcow2 image through "query-block" qmp command.
>
> libxl_snapshot = Struct("snapshot",[
>     ("device",        string),
>   

Should this be libxl_device_disk instead of a string? And should
multiple be supported, allowing control over which of a domain's disks
are included in the snapshot?

>     ("name",          string),
>   

"description" would be helpful.

>     ("id",            string),
>     ("vm_state_size", uint64),
>     ("date_sec",      uint64),
>     ("date_nsec",     uint64),
>     ("vm_clock_sec",  uint64),
>     ("vm_clock_nsec", uint64),
>   

A lot of these are read-only and wouldn't be provided when calling
libxl_snapshot_create(). I think adding the domain state at time of
snapshot would be useful. There are probably other items I'm not
considering atm, which makes me nervous about future extensibility.

> (2), libxl_vm_snapshot store vm snapshot information which store in the path
> shown above. i add some api for create, delete and list these information.
> at first, i want to add these information to xenstore, but it will lose when
> xenstore reboot or dom0 reboot.
>
> libxl_vm_snapshot = Struct("vm_snapshot",[
>     ("name",          string),
>     ("creation_time", uint64),
>     ("save",          string),
>     ])
>
> 2), new api
> (1), in libxl/libxl.h
> /* disk snapshot api
>  * support create, delete and list for internal snapshot of a single disk
>  * only support internal snapshot rigt now.
>  */
> /* create disk snapshot according to the device name in snapshot array. nb is
>  * the number of snapshot array.
>  * use the qmp transaction to ensure all snapshot of disk is coherence.
>  */
> int libxl_disk_snapshot_create(libxl_ctx *ctx, int domid,
>                                libxl_snapshot *snapshot, int nb);
> /* delete number of nb disk snapshot describe in snapshot array
>  */
> int libxl_disk_snapshot_delete(libxl_ctx *ctx, int domid,
>                                libxl_snapshot *snapshot, int nb);
> int libxl__disk_snapshot_delete(libxl_ctx *ctx, int domid,
>                                 libxl_snapshot *snapshot);
> /* apply the disk snapshot by qemu-img command
>  */
> int libxl_disk_snapshot_apply(libxl_ctx *ctx, int domid,
>                               libxl_snapshot *snapshot, int nb);
> /* list disk snapshot by qmp query-block command
>  */
> int libxl__disk_snapshot_list(libxl_ctx *ctx, int domid,
>                               libxl_snapshot **snapshotp);
>
> /* vm snapshot api
>  */
> /* write vm snapshot information to file
>  */
> int libxl_vm_snapshot_add(libxl_ctx *ctx, uint32_t domid,
>                           libxl_vm_snapshot *snapshot,
>                           const libxl_asyncop_how *ao_how)
>                           LIBXL_EXTERNAL_CALLERS_ONLY;
> /* delete vm snapshot information file
>  */
> int libxl_vm_snapshot_remove(libxl_ctx *ctx, uint32_t domid,
>                              libxl_vm_snapshot *snapshot,
>                              const libxl_asyncop_how *ao_how)
>                              LIBXL_EXTERNAL_CALLERS_ONLY;
> /* list vm snapshot from file
>  */
> libxl_vm_snapshot *libxl_vm_snapshot_list(libxl_ctx *ctx, uint32_t domid,
>                                           int *num)
>                                           LIBXL_EXTERNAL_CALLERS_ONLY;
> /* read vm snapshot information from dedicated file
>  */
> int libxl_vm_snapshot_getinfo(libxl_ctx *ctx, uint32_t domid,
>                               libxl_vm_snapshot *snapshot);
> /* delete vm save image */
> int libxl_vm_snapshot_delete_save_image(libxl_ctx *ctx,
>                                         libxl_vm_snapshot *snapshot);
>   

IMO, these too should be combined into libxl_snapshot_*(), with disk vs
domain snapshot controlled via a flags argument.

> (2), libxl/xl_cmdimpl.c
> /* get disk device name (hda, hdc..) and image path through
>  * libxl_device_disk_list and libxl_device_disk_getinfo
>  */
> static int get_disk(uint32_t domid, libxl_snapshot **snapshotp, char *name);
>
> 3, question
> 1), how to do disk snapshot apply?
> when i apply the vm snapshot, i need to revert the disk snapshot. in qemu, it is
> done by bdrv_snapshot_goto api. this api is only called by loadvm hmp or
> qemu-img commands.
> there is no hmp api in libxl. so the only choice is using qemu-img command.
> i know usually i should call qmp for qemu operation. but there is not qmp at
> the moment, could i call qemu-img in libxl for disk snapshot apply?
>   

Similar to the libvirt qemu driver, is it possible to use both qmp and
hmp monitors in libxl?

Regards,
Jim