Understanding the underlying components of SFW's Quick Recovery process

SFW's Quick Recovery solution uses FlashSnap and FastResync technology to leverage the Microsoft Volume Shadow Copy Service (VSS) capability to pause and resume a VSS-aware application.


FlashSnap provides the ability to create and maintain the on-host point-in-time copies that are integral to the Quick Recovery solution. FlashSnap is the multi-step process used to create and maintain split-mirror snapshots that are copies of the original volumes they mirror. Both the original and snapshot volumes may consist of multiple physical devices, as in the case of RAID 0+1 (mirrored striped) volumes. FlashSnap cannot be used with software RAID-5 volumes.

FlashSnap includes the following commands:

These FlashSnap commands are implemented through the SFW GUI. CLI equivalents of these GUI commands exist, and are available as parameters to the vxassist or vxsnap command.

FastResync (FR)

The FastResync capability optimizes the resynchronization of a snapshot volume and its original volume. FlashSnap uses FastResync technology to track the changed blocks in an original volume after a snapshot is detached. When the snapshot volume is resynchronized with the original volume by using the Snap Back command, only the changed data blocks are written to the snapshot volume. This greatly reduces the time and performance impact of resynchronization, which means that a Quick Recovery image can be refreshed with minimal impact on production.

FR is automatically enabled for a volume when the prepare operation is performed on the volume through the GUI Prepare command or the CLI vxassist snapstart command.

Microsoft Volume Shadow Copy Service (VSS)

Microsoft Volume Shadow Copy Service (VSS) is a Windows service that provides the capability of creating snapshots or volume shadow copies. A volume shadow copy is a volume that represents a duplicate of the state of the original volume at the time the copy began. SFW integrates VSS into its snapshot function through the vxsnap command. Because SFW is a VSS requestor, it can initiate VSS snapshots at any time.

The vxsnap command makes use of both FlashSnap and VSS technology to create high-quality snapshots that can be done when application files are open. VSS can quiesce the application for the moment when the snapshot is created and then resume the application immediately after the snapshot; but a VSS-aware application must be used, such as Microsoft SQL Server.

For more information on how VSS and SFW work together, see the Storage Foundation Administrator's Guide.