The VMware VSS Implementation

As of Windows Server 2008, disk UUIDs must be enabled for VSS quiesced snapshots. Disk UUIDs might not be enabled if a virtual machine was upgraded from virtual hardware version 4.

VMware VSS does not support virtual machines with IDE disks, nor does it support virtual machines with an insufficient number of free SCSI slots.

Before vSphere 5.1, reverting to a writable snapshot sometimes left orphaned virtual disks that the system never removed. In the vSphere 5.1 release, writable snapshots are correctly accounted for as sibling snapshots. This permits cleaner management, because the disk chain matches the snapshot hierarchy, and it avoids orphaned disks. Linux backup software takes a read-only snapshot so is not affected. On Windows, VSS backup software may create two snapshots, one made writable by calling CreateSnapshot_task with the quiesce flag set true.

To add support for granular application control, specify:

• whether pre-freeze and post-thaw scripts get invoked
• whether quiescing gets invoked
• VSS snapshot context (application, file system quiescing, and so forth)
• VSS backup context (full, differential, incremental)
• writers/components to be involved during quiescing
• whether to fail quiescing or continue if one of the writers fails to quiesce
• retry count

A VSS quiesced snapshot reports as VSS_BT_COPY to VSS, hence no log truncation. The VSS manifest can be downloaded with HTTP. By default, all VSS writers are involved, but a mechanism for excluding writers exists; see the VMware KB article 1031200. For help troubleshooting, see KB article 1007696.