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The more VBDs attached to a VM, the more likely it is that this timeout may be reached. Citrix recommends attaching no more that 2 VBDs to a VM to avoid reaching the timeout. However, there is a workaround to this problem. The probability of taking a successful VSS based snapshot of a VM with more than 2 VBDs can be increased manifold, if all the VDIs for the VM are hosted on different SRs.

VSS snapshot all the disks attached to a VM: in order to store all data available at the time of a VSS snapshot, the XAPI manager will snapshot all disks and the VM metadata associated with a VM that can be snapshotted using the XenServer storage manager API. If the VSS layer requests a snapshot of only a subset of the disks, a full VM snapshot will not be taken.

vm-snapshot-with-quiesceproduces bootable snapshot VM images: To achieve this, the XenServer VSS hardware provider makes snapshot volumes writable, including the snapshot of the boot volume.

VSS snap of volumes hosted on dynamic disks in the Windows Guest : The vm-snapshot-with-quiesce CLI and the XenServer VSS hardware provider do not support snapshots of volumes hosted on dynamic disks on the Windows VM.

Note:

Using EqualLogic or NetApp storage requires a Citrix XenServer Advanced Edition or higher license. To learn more about XenServer editions, and to find out how to upgrade, visit the Citrix website here.

Note:

Do not forget to install the Xen VSS provider in the Windows guest in order to support VSS. This is done using the install-XenProvider.cmdscript provided with the Windows PV drivers. More details can be found in the Virtual Machine Installation Guide in the Windows section.

In general, a VM can only access VDI snapshots (not VDI clones) of itself using the VSS interface. There is a flag that can be set by the XenServer administrator whereby adding an attribute of snapmanager=true to the VM's other-configallows that VM to import snapshots of VDIs from other VMs.

Warning:

This opens a security vulnerability and should be used with care. This feature allows an administrator to attach VSS snapshots using an in-guest transportable snapshot ID as generated by the VSS layer to another VM for the purposes of backup.

VSS quiesce timeout: the Microsoft VSS quiesce period is set to a non-configurable value of 10 seconds, and it is quite probable that a snapshot may not be able to complete in time. If, for example the XAPI daemon has queued additional blocking tasks such as an SR scan, the VSS snapshot may timeout and fail. The operation should be retried if this happens.

Note:

The more VBDs attached to a VM, the more likely it is that this timeout may be reached. Citrix recommends attaching no more that 2 VBDs to a VM to avoid reaching the timeout. However, there is a workaround to this problem. The probability of taking a successful VSS based snapshot of a VM with more than 2 VBDs can be increased manifold, if all the VDIs for the VM are hosted on different SRs.

VSS snapshot all the disks attached to a VM: in order to store all data available at the time of a VSS snapshot, the XAPI manager will snapshot all disks and the VM metadata associated with a VM that can be snapshotted using the XenServer storage manager API. If the VSS layer requests a snapshot of only a subset of the disks, a full VM snapshot will not be taken.

vm-snapshot-with-quiesce produces bootable snapshot VM images : To achieve this, the XenServer VSS hardware provider makes snapshot volumes writable, including the snapshot of the boot volume.

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