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Snapshots and the volume shadow copy service (VSS)

Snapshots of storage attached to the storage server may be initiated by Microsoft’s Volume Shadow Copy Service (VSS). VSS is available on servers running Windows Server 2003 or 2008, and it provides a coordination point for enabling consistent backups of online storage. Snapshots initiated by VSS are exported as iSCSI LUNs.

Storage writers (for example MS Exchange or a backup application) first register with VSS. A VSS credential is saved on both the VSS host and the NAS server. The server address and port number are saved along with the credential. This means that if the NAS server's VSS management port setting is changed, any existing VSS credentials for that server must be removed and new credentials must be created. If a DNS name is used for a NAS server, then changes to the server's IP address alone will not require removing and recreating the credential.

NoteA VSS credential has limited rights on the server: it can only be used to perform VSS-related operations using the VSS management interface. In particular it cannot be used to gain access to the normal NAS server management console, either locally or remotely.

Then, when a backup application wishes to back up a piece of storage (a “volume”):

  1. The backup application requests that VSS take the snapshot.

  2. VSS requests that all registered writers flush their data to make sure that all of their on-disk data files are in a consistent state.

  3. After the writers report completion of this step, VSS takes the snapshot.

    When a VSS initiated snapshot is taken of a file system, the snapshot is added to the iSCSI target as one or more iSCSI LUs (one iSCSI LU is added for each source LU supplied to VSS). The snapshot LUs are then visible to the VSS host (the system on which the VSS Hardware Provider is installed) and are used as the backup source.

    NoteIf a VSS snapshot request contains LUs on different file systems, then only one snapshot will be created for all the LUs in each file system. However, copies of each requested LU are always created and made visible to the VSS host by the NAS server.
  4. VSS then returns a pointer to the snapshot to the backup application so that the backup application can back up a stable view of the storage (the snapshot).

  5. After the backup is completed, the backup application notifies VSS so that the snapshot may be deleted.

    For non-persistent snapshots, once the backup is complete, the snapshot LUs are removed from the target and the VSS initiated snapshot(s) are deleted.

    Persistent snapshots should be deleted via the backup application whenever possible. Although it is possible to delete a VSS initiated snapshot via the CLI or NAS Manager, care must be taken to ensure that a backup application is not active and an iSCSI host is not bound to the snapshot's LU(s). Properly deleting a snapshot will also result in the snapshot LUs being removed from the target.

    NoteUsing the CLI or NAS Manager to delete VSS initiated snapshots or to remove the snapshot LUs from their associated iSCSI target will result in the unexpected removal of a disk from the VSS host system, and can cause the VSS host to crash.

VSS may also be used to take “point in time” copies for later reference. The process is similar, except in this case no automatic deletion of the snapshot is performed by VSS. The storage server supports this mechanism by means of a VSS “hardware provider,” a DLL which registers with VSS in order to support snapshots of volumes attached to a storage server.

NoteSnapshots initiated by the VSS service only contain images of iSCSI LUNs attached to the storage server. Non-iSCSI volumes attached to the storage server are not included in snapshots initiated by VSS.

 

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