Snapshots

Snapshots represent the state of a VM, or a disk (VDI) at a point in time. They can be used for:

  • backups (hourly, daily, weekly etc)
  • experiments (take snapshot, try something, revert back again)
  • golden images (install OS, get it just right, clone it 1000s of times)

Read more about Snapshots: the High-Level Feature.

Taking a VDI snapshot

To take a snapshot of a single disk (VDI):

snapshot_vdi <- VDI.snapshot(session_id, vdi, driver_params)

where vdi is the reference to the disk to be snapshotted, and driver_params is a list of string pairs providing optional backend implementation-specific hints. The snapshot operation should be quick (i.e. it should never be implemented as a slow disk copy) and the resulting VDI will have

Field name Description
is_a_snapshot a flag, set to true, indicating the disk is a snapshot
snapshot_of a reference to the disk the snapshot was created from
snapshot_time the time the snapshot was taken

The resulting snapshot should be considered read-only. Depending on the backend implementation it may be technically possible to write to the snapshot, but clients must not do this. To create a writable disk from a snapshot, see “restoring from a snapshot” below.

Note that the storage backend is free to implement this in different ways. We do not assume the presence of a .vhd-formatted storage repository. Clients must never assume anything about the backend implementation without checking first with the maintainers of the backend implementation.

Restoring to a VDI snapshot

To restore from a VDI snapshot first

new_vdi <- VDI.clone(session_id, snapshot_vdi, driver_params)

where snapshot_vdi is a reference to the snapshot VDI, and driver_params is a list of string pairs providing optional backend implementation-specific hints. The clone operation should be quick (i.e. it should never be implemented as a slow disk copy) and the resulting VDI will have

Field name Description
is_a_snapshot a flag, set to false, indicating the disk is not a snapshot
snapshot_of an invalid reference
snapshot_time an invalid time

The resulting disk is writable and can be used by the client as normal.

Note that the “restored” VDI will have a different VDI.uuid and reference to the original VDI.

Taking a VM snapshot

A VM snapshot is a copy of the VM metadata and a snapshot of all the associated VDIs at around the same point in time. To take a VM snapshot:

snapshot_vm <- VM.snapshot(session_id, vm, new_name)

where vm is a reference to the existing VM and new_name will be the name_label of the resulting VM (snapshot) object. The resulting VM will have

Field name Description
is_a_snapshot a flag, set to true, indicating the VM is a snapshot
snapshot_of a reference to the VM the snapshot was created from
snapshot_time the time the snapshot was taken

Note that each disk is snapshotted one-by-one and not at the same time.

Restoring to a VM snapshot

A VM snapshot can be reverted to a snapshot using

VM.revert(session_id, snapshot_ref)

where snapshot_ref is a reference to the snapshot VM. Each VDI associated with the VM before the snapshot will be destroyed and each VDI associated with the snapshot will be cloned (see “Reverting to a disk snapshot” above) and associated with the VM. The resulting VM will have

Field name Description
is_a_snapshot a flag, set to false, indicating the VM is not a snapshot
snapshot_of an invalid reference
snapshot_time an invalid time

Note that the VM.uuid and reference are preserved, but the VDI.uuid and VDI references are not.

Downloading a disk or snapshot

Disks can be downloaded in either raw or vhd format using an HTTP 1.0 GET request as follows:

GET /export_raw_vdi?session_id=%s&task_id=%s&vdi=%s&format=%s[&base=%s] HTTP/1.0\r\n
Connection: close\r\n
\r\n
\r\n

where

  • session_id is a currently logged-in session
  • task_id is a Task reference which will be used to monitor the progress of this task and receive errors from it
  • vdi is the reference of the VDI into which the data will be imported
  • format is either vhd or raw
  • (optional) base is the reference of a VDI which has already been exported and this export should only contain the blocks which have changed since then.

Note that the vhd format allows the disk to be sparse i.e. only contain allocated blocks. This helps reduce the size of the download.

The xapi-project/xen-api repo has a python download example

Uploading a disk or snapshot

Disks can be uploaded in either raw or vhd format using an HTTP 1.0 PUT request as follows:

PUT /import_raw_vdi?session_id=%s&task_id=%s&vdi=%s&format=%s HTTP/1.0\r\n
Connection: close\r\n
\r\n
\r\n

where

  • session_id is a currently logged-in session
  • task_id is a Task reference which will be used to monitor the progress of this task and receive errors from it
  • vdi is the reference of the VDI into which the data will be imported
  • format is either vhd or raw

Note that you must create the disk (with the correct size) before importing data to it. The disk doesn’t have to be empty, in fact if restoring from a series of incremental downloads it makes sense to upload them all to the same disk in order.

Example: incremental backup with xe

This section will show how easy it is to build an incremental backup tool using these APIs. For simplicity we will use the xe commands rather than raw XMLRPC and HTTP.

For a VDI with uuid $VDI, take a snapshot:

FULL=$(xe vdi-snapshot uuid=$VDI)

Next perform a full backup into a file “full.vhd”, in vhd format:

xe vdi-export uuid=$FULL filename=full.vhd format=vhd  --progress

If the SR was using the vhd format internally (this is the default) then the full backup will be sparse and will only contain blocks if they have been written to.

After some time has passed and the VDI has been written to, take another snapshot:

DELTA=$(xe vdi-snapshot uuid=$VDI)

Now we can backup only the disk blocks which have changed between the original snapshot $FULL and the next snapshot $DELTA into a file called “delta.vhd”:

xe vdi-export uuid=$DELTA filename=delta.vhd format=vhd base=$FULL --progress

We now have 2 files on the local system:

  • “full.vhd”: a complete backup of the first snapshot
  • “delta.vhd”: an incremental backup of the second snapshot, relative to the first

For example:

test $ ls -lh *.vhd
-rw------- 1 dscott xendev 213M Aug 15 10:39 delta.vhd
-rw------- 1 dscott xendev 8.0G Aug 15 10:39 full.vhd

To restore the original snapshot you must create an empty disk with the correct size. To find the size of a .vhd file use qemu-img as follows:

test $ qemu-img info delta.vhd
image: delta.vhd
file format: vpc
virtual size: 24G (25769705472 bytes)
disk size: 212M

Here the size is 25769705472 bytes. Create a fresh VDI in SR $SR to restore the backup as follows:

SIZE=25769705472
RESTORE=$(xe vdi-create name-label=restored virtual-size=$SIZE sr-uuid=$SR type=user)

then import “full.vhd” into it:

xe vdi-import uuid=$RESTORE filename=full.vhd format=vhd --progress

Once “full.vhd” has been imported, the incremental backup can be restored on top:

xe vdi-import uuid=$RESTORE filename=delta.vhd format=vhd --progress

Note there is no need to supply a “base” parameter when importing; Xapi will treat the “vhd differencing disk” as a set of blocks and import them. It is up to you to check you are importing them to the right place.

Now the VDI $RESTORE should have the same contents as $DELTA.

Related XenAPI Classes, Fields and Messages

Classes

NameDescription

Fields

NameDescription
VDI.is_a_snapshot true if this is a snapshot.
VDI.snapshot_of Ref pointing to the VDI this snapshot is of.
VDI.snapshot_time Date/time when this snapshot was created.
VDI.snapshots List pointing to all the VDIs snapshots.

Messages

NameDescription
VM.revertReverts the specified VM to a previous state.
VM.snapshotSnapshots the specified VM, making a new VM. Snapshot automatically exploits the capabilities of the underlying storage repository in which the VM's disk images are stored (e.g. Copy on Write).
VDI.cloneTake an exact copy of the VDI and return a reference to the new disk. If any driver_params are specified then these are passed through to the storage-specific substrate driver that implements the clone operation. NB the clone lives in the same Storage Repository as its parent.
VDI.snapshotTake a read-only snapshot of the VDI, returning a reference to the snapshot. If any driver_params are specified then these are passed through to the storage-specific substrate driver that takes the snapshot. NB the snapshot lives in the same Storage Repository as its parent.