LUN masking makes for fast data recovery

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your system loses data, your first priority is to recover quickly and get back in operation. Yet you probably want to examine any corrupted data, too, which will help you diagnose the problem or possibly recover anything that wasn't covered by the last backup.

While these two tasks may seem mutually contradictory, Windows Server 2003 provides a way to do both.

Using the Volume Shadow Copy Service and Virtual Disk Service built into Windows Server 2003, you can set system restore points by creating snapshots of your system. On a storage area network (SAN), these features can be used to create images of the system, including Active Directory, which can be restored almost instantly.

The trick lies in the use of LUN (logical unit number) masking. Although SAN storage is physically available to all of the servers on the SAN, the servers can only access the LUNs that are unmasked to each server. When Volume Shadow Copy Service creates a system snapshot, it uses the Virtual Disk Service's Diskraid utility's mask command to mask off the LUN containing the snapshot from the server being backed up. It does that so the copy will not confuse the server. In the event of a failure, the LUN containing the failed storage can be masked off and the snapshot can be unmasked. By manipulating the LUNs, it appears that the damaged copy of the data is transported out of the server and the backup snapshot is transported in, even though the data is not actually moved.

Microsoft describes the process in some detail in a document, "Windows Server 2003 Active Directory Fast Recovery with Volume Shadow Copy Service and Virtual Disk Service."

Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years he has been a freelance writer specializing in storage and other computer issues.

This was first published in August 2005

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