Hi all. Purpose of this API is to help with making host-side backups of running domains without having to suspend them until backup finishes. The API can be used to create a consistent snapshot of a disk assigned to a domain, which can be later used for making backups. For better consistency, libvirt will try to notify the domain to make data on disk consistent. Guest agent is needed for this to work and libvirt will talk with it using hypervisor API or directly. Libvirt already provides snapshot support with virDomainSnapshot* APIs but they serve a bit different purpose. Domain snapshots consist of current state of all disks and runtime state (if the domain is running) taken at the same time. Since memory is also saved, domain snapshot creation can take some time during which the domain may need to be suspended. The main benefit of these APIs is that they can be used to create checkpoints of a domain in a known good state to which the domain can later be reverted. Disk snapshot API needs to be general and flexible enough to support various storage and snapshot methods. Examples of what can be used for creating disk snapshots are: QEMU qcow2 snapshots, LVM snapshots, filesystems with snapshot support (btrfs, zfs, ...), enterprise storage. Moreover, method used to create disk snapshot does not have to be determined by disk type. One can have a qcow2 disk stored on btrfs inside lvm logical volume; snapshot of such disk can be done at any of the three levels. Hypervisor support: QEMU supports snapshot_blkdev monitor command for creating qcow2 snapshots. VMware does not seem to support per-device snapshots, only per-VM snapshot without memory. VirtualBox provides IMedium::createDiffStorage(), IMedium::mergeTo() for per-device snapshots but they do not seem to work on disks in use. However, full support from hypervisors is not strictly required since snapshots can be done outside of them although at a possible cost of losing consistency. As different snapshot methods may require different input data, new diskSnapshot XML element is introduced to describe disk snapshot method: <diskSnapshot method='qemu|lvm|btrfs|...'> <!-- required data depending on the method --> </diskSnapshot> For qemu method, the element can be very simple: <diskSnapshot method='qemu'/> For lvm, logical volume device needs to be specified in case it is not the same as disk to be snapshotted: <diskSnapshot method='lvm'> <device path='/dev/vg/lv4'/> </diskSnapshot> Enterprise storage would need their own ways of identifying volumes to be snapshotted. The diskSnapshot element can be used inside disk element in domain XML: <domain ...> ... <devices> ... <disk ...> ... <diskSnapshot ...> ... </diskSnapshot> </disk> </devices> </domain> The "disk" part of "diskSnapshot" element name may seem to be redundant but I wanted to avoid confusion with snapshot XML element used for domain snapshots. Existing virDomainUpdateDevice API can be used to alter snapshot method on existing disk devices. 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 @name snapshot name @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 @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 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. Jirka