The VSS framework


The VSS framework
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There are four components to the VSS framework: Requestor, Writer, Provider, and the Volume Shadow Copy Service itself.

Table: VSS framework components

Component	Action
Volume Shadow Copy Service	Talks to and coordinates the Requestor, Provider, and Writer.
Requestor	As a Requestor, the vxsnap component of FlashSnap notifies the VSS coordinator to initiate the VSS request to prepare Exchange for quiescing and later requests that the snapshot process begin.
Writer	As Writers, VSS-enabled applications such as Exchange respond to requests to prepare and participate in the generation of snapshots, provide success/failure status, and provide information about the application including what is to be backed up and restored, and restore strategy. The Exchange 2010 VSS Writer has two instances, the Store Writer and the Replication Writer. The Store Writer is available on any mailbox server and is responsible for backup/restore of active databases. The Replication Writer is available on the passive node of a DAG server. The Replication Writer supports backup functionality for a selected database where the snapshot is taken against the replicated instance of the database and transaction log files. The Replication Writer is supported by SFW Flashsnap for snapshot and schedule snapshot operations but not for restore operations.
Provider	As a Provider, FlashSnap creates the persistent snapshot.

The following figure shows the steps in the Volume Shadow Copy Service Process.

Figure: The Volume Shadow Copy Service process

FlashSnap integrates with Volume Shadow Copy Service as both a VSS Requestor and a VSS Provider.

The following steps occur, in the order presented, during the snapshot process:

Acting as a VSS Requestor, FlashSnap notifies the VSS coordinator service to prepare for a split-mirror snapshot of an Exchange database.
The VSS coordinator service calls the Exchange VSS Writer to find out which volumes contain the databases and transaction logs.
The VSS coordinator service notifies the FlashSnap VSS Provider to prepare for the snapshot.
Acting as a VSS Requestor, FlashSnap requests that the VSS coordinator service begin the snapshot call.
The VSS coordinator service notifies the Exchange VSS Writer to quiesce and freeze the databases in preparation for the snapshot. When this is accomplished, the Exchange Writer informs the VSS coordinator service to proceed.
The VSS coordinator service calls the FlashSnap Provider to create the split-mirror snapshot by detaching the snapshot volume from the original volume. The snapshot process takes a maximum of 10 seconds. After the snapshot volume is detached, the FlashSnap Provider informs the VSS coordinator service to proceed.
The VSS coordinator service notifies the Exchange Writer to thaw (release the freeze) and resume normal I/O.

The following steps occur, in the order presented, during the restore process:

Acting as a VSS Requestor, FlashSnap notifies the VSS coordinator service to prepare for a restore operation.
The VSS coordinator service calls the Exchange VSS Writer, which prepares for the restore operation.
The FlashSnap utility restores the snapshot volumes. After the snapback operation completes, the FlashSnap utility informs the VSS coordinator service to proceed.
The VSS coordinator service notifies the Exchange Writer to process the post-restore operations.