Shadow Sets

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Microsoft is building VSS into its latest servers. Shadow copies of user file changes are stored and available to the user, so when users accidentally delete something, they can retrieve it simply by looking up the shadow copies on the server. These copies, in fact, contain only the actual changes made to files so they have a lower footprint; that is, this is a copy-on-write approach to snapshots, not split mirroring. A shadow copy is essentially a snapshot of the existing state of a working volume. Instead of copying every single bit, it is simply a log of changes at a specific point in time, changes made since the last system snapshot was taken.

The Windows Server 2003 family included VSS and many other improved management and storage functions. The Microsoft Management Console (MMC) is still used as the main management interface and incorporates Manage Your Server, a role-based management interface that organizes administrative tasks much better than previously. According to initial reports, it is simpler and more user friendly than the Configure Your Server interface of Windows 2000. Among the new core features of Windows Server 2003, storage has advanced the most. At a low level, the new operating system includes support for SANs and provides a unified management interface (including a new API) for hardware RAID devices. The most significant functional enhancements relate to shadow copies and federated file servers.

Advantages of VSS

When client versions of VSS are installed on remote desktops, users can recover lost data via the snapshots as the shadow copies are exposed to network users. Thus, when a document is accidentally deleted, the user goes into Windows Explorer, right-clicks the folder, and then selects the date and time of the snapshot from a list. Voila — the data is recovered without the user calling the help desk or requesting IT's assistance. Lost files can be recovered in moments, and the snapshot itself on even large drives takes about a minute without impacting server availability. Further, shadow copies of volumes are made at the time a backup is initiated. Data can then be backed up from the shadow copy instead of from the original volume. The original volume continues to change as the process continues, but the shadow copy of the volume remains constant. Users can refer back to earlier shadow copies, thereby retrieving earlier versions of documents that have been lost.

Volume ShadowCopy Service technology has many advantages. Servers and workstations can be backed up while applications and services are running, and open files are not skipped during the backup process. More importantly, the need to schedule a backup window where applications must be shut down is removed. VSS is much more thorough than System Restore. It completes data file snapshots so all the data is protected. As noted previously, it allows multiplatform interoperation and cooperation among storage hardware, storage software, and business applications; lets vendors plug in snapshot, clone, and data replication solutions; and allows integration with other applications. One warning, though: VSS and other approaches to snapshots must have temporary disk space available; if space is not available, VSS skips open files. This is another reason why you should always ensure your disks contain plenty of free space. Additionally, by virtue of the fact that snapshots make many temporary copies of a volume, the level of fragmentation is greatly increased. So, take advantage of snapshot technology, by all means, but be sure to support it with regular defragmentation.

One further warning is that VSS can be a space and resource hog. Default settings allocate as much as 12 percent of a volume to snapshots, and it runs many times during the day, thus consuming a significant amount of CPU time and RAM. It is probably best to select as little snapshot room as the system will allow, and do not expect to be able to go back 9 months to find a lost file. At best, a library of a month or two of files and file changes can be accessed.

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