HP StorageWorks 4000s NAS 4000s and 9000s Administration Guide - Page 93

Shadow Copies 5 Overview Note: The NAS 4000s and 9000s servers can be deployed in a clustered as well as a non-clustered configuration. This chapters discusses using Shadow Copies in a non-clustered environment. Please review the Cluster Administration chapter of this guide for additional information regarding Shadow Copies in a cluster. The Volume Shadow Copy Service provides an infrastructure for creating point-in-time snapshots (shadow copies) of volumes. Shadow Copy supports 64 shadow copies per volume. A shadow copy contains previous versions of the files or folders contained on a volume at a specific point in time. While the Shadow Copy mechanism is managed at the server (see the "Managing Shadow Copy" section in this chapter), previous versions of files and folders are only available over the network from clients and are seen on a per folder or file level and not as an entire volume. The Shadow Copy feature works at the block level. As changes are made to the file system, the Shadow Copy Service copies out the original blocks to a special cache file, to maintain a consistent view of the file at a particular point in time. Since the snapshot only contains a subset of the original blocks, the cache file is typically smaller than the original volume. In the snapshot's original form, it takes up no space since blocks are not moved until an update to the disk occurs. By using shadow copies, a NAS server can maintain a set of previous versions of all files on the selected volumes. End users access the file or folder by using a separate client add-on program, which enables them to view the file in Windows Explorer. Accessing previous versions of files, or shadow copies, enables users to: ■ Recover files that were accidentally deleted. Previous versions can be opened and copied to a safe location. ■ Recover from accidentally overwriting a file. A previous version of that file can be accessed. ■ Compare several versions of a file while working. Use previous versions to compare changes between two versions of a file. Shadow copies cannot replace the current backup, archive, or business recovery system, but they can help to simplify restore procedures. Since a snapshot only contains a portion of the original data blocks, shadow copies can not protect against data loss due to media failures. However the strength of snapshots is the ability to instantly recover data from shadow copies, reducing the number of times needed to restore data from tape. NAS 4000s and 9000s Administration Guide 93