Improving snapshot revert behaviour

Design document
Revision v1
Status confirmed
Review create new issue

Currently there is a XenAPI VM.revert which reverts a “VM” to the state it was in when a VM-level snapshot was taken. There is no VDI.revert so VM.revert uses VDI.clone to change the state of the disks.

The use of VDI.clone has the side-effect of changing VDI refs and uuids. This causes the following problems:

  • It is difficult for clients such as Apache CloudStack to keep track of the disks it is actively managing
  • VDI snapshot metadata (VDI.snapshot_of et al) has to be carefully fixed up since all the old refs are now dangling

We will fix these problems by:

  1. adding a VDI.revert to the SMAPIv2 and calling this from VM.revert
  2. defining a new SMAPIv1 operation vdi_revert and a corresponding capability VDI_REVERT
  3. the Xapi implementation of VDI.revert will first try the vdi_revert, and fall back to VDI.clone if that fails
  4. implement vdi_revert for common storage types, including File and LVM-based SRs.

XenAPI changes

We will add the function VDI.revert with arguments:

  • in: snapshot: Ref(VDI): the snapshot to which we want to revert
  • in: driver_params: Map(String,String): optional extra parameters
  • out: Ref(VDI) the new VDI

The function will look up the VDI which this is a snapshot_of, and change the VDI to have the same contents as the snapshot. The snapshot will not be modified. If the implementation is able to revert in-place, then the reference returned will be the VDI this is a snapshot_of; otherwise it is a reference to a fresh VDI (created by the VDI.clone fallback path)


SMAPIv1 changes

We will define the function vdi_revert with arguments:

  • in: sr_uuid: the UUID of the SR containing both the VDI and the snapshot
  • in: vdi_uuid: the UUID of the snapshot whose contents should be duplicated
  • in: target_uuid: the UUID of the target whose contents should be replaced

The function will replace the contents of the target_uuid VDI with the contents of the vdi_uuid VDI without changing the identify of the target (i.e. name-label, uuid and location are guaranteed to remain the same). The vdi_uuid is preserved by this operation. The operation is obvoiusly idempotent.

Xapi changes

Xapi will

  • use VDI.revert in the VM.revert code-path
  • expose a new xe vdi-revert CLI command
  • implement the VDI.revert by calling the SMAPIv1 function and falling back to VDI.clone if a Not_implemented exception is thrown


SM changes

We will modify

  • and to add a new vdi_revert function which throws a ‘not implemented’ exception
  • to implement VDI.revert using a variant of the existing snapshot/clone machinery
  • and to advertise the VDI_REVERT capability
  • to implement VDI.revert using a variant of the existing snapshot/clone machinery
  • and to advertise the VDI_REVERT capability

Prototype code

Prototype code exists here: