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

["Chun Yan Liu" <cyliu@xxxxxxxx>]

>>> On 12/17/2014 at 10:09 PM, in message
<20141217140958.GH1904@xxxxxxxxxxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>
wrote: 
> 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? 
Right.

> 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) 

Libvirt has its own data structure to manage domain snapshots. 
libxl_disk_snapshot
is only used by libvirt to call libxl API to do disk snapshot work for it. In 
libvirt,
it has other data structure to represent disk snapshot information.


>  
> >  
> > 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. 

Thanks. That's better.

>  
> >  
> > /*  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. 

OK. Thanks. Then we can put all disk snapshot APIs in libxl.

>  
> 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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