Re: [libvirt] [RFC] Live disk snapshot API

On Thu, Jan 06, 2011 at 02:27:14PM +0100, Jiri Denemark wrote: > To create a snapshot of a disk, the following API is introduced: > > int > virDomainDiskSnapshotCreate(virDomainPtr domain, > const char *disk, > const char *method, > const char *name, > char **modifiedDisk, > char **backupSource, > unsigned int flags); > @domain > pointer to domain object > @disk > XML definition of the disk to snapshot > @method > optional <diskSnapshot> XML element overriding the one from <disk> > element; if none of them is specified, default method according to disk > type (and pool) is used; e.g., qcow disk => qemu method; logical volume in > LVM pool => lvm method This I'd consider mandatory > @name > snapshot name What is the 'name' in this context ? In case of QCow2, AFAICT, the only identifier that really matters would be the filename, which is surely part of the <diskSnapshot> XML passed into @method. Likewise for non-QCow@ types, wouldn't @method already have the unique identifiers you need ? > @modifiedDisk > place where to store modified XML description of the disk which the domain > is now using; NULL is stored if domain is still using the original disk > (snapshot was created separately) > @backupSource > place where to store 'source' element from disk XML describing the disk > which can be used to take backups of 'disk' (i.e., read-only and immutable > snapshot); it might either be the same as provided in 'disk' or something > else (depending on method/implementation used); e.g., for qemu method, the > element describes previous disk source; lvm creates a new device for > snapshot and keeps writing into the original device At which point neither of these really have any good reason to exist. > @flags > OR'ed set of flags: > - VIR_DOMAIN_DISK_SNAPSHOT_QUIESCE_REQUIRED -- if no guest agent is > running/answering requests for consistent disk state, fail the API; > otherwise, the snapshot will be done regardless If the guest has multiple disks, then apps will need to iterate over each disk, invoking virDomainDiskSnapshotCreate for each one. If they do that, then there's a reasonable chance they'll want to do the quiesce once upfront, so all disks are consistent wrt each other. This kind of suggests we want a virDomainQuiesceStorage() and virDomainUnquiesceStorage() API pair. > I have a slight feeling that the API is a bit over-engineered but I'm not > entirely sure if it can be simplified and still provide the flexibility and > future-compatibility. I have this feeling especially about backupSource output > parameter which might possibly replaced with a simple char * (eventually > returned directly by the API instead of int) containing file/device path. > Another think which is not strictly needed is modifiedDisk. The caller can ask > for domain XML and look the device there if needed but that would be quite > complicated. Thus returning it from this API seemed useful and logical too, > since the API is possibly changing disk XML and it make sense to return the > changes. > > Deleting/merging snapshots previously created by virDomainDiskSnapshotCreate > is not covered by this proposal and will need to be added in the future to > complete disk snapshot support. As long as the new files created via this are all within scope of a storage pool, then they can be deleted that way. Merging snapshots is also probably something that'd probably want to be done via the storage pool APIs.