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Snapshots and Clones for Data Recovery

The subject of snapshots and clones for duplicating data comes up a lot, and the topic is somewhat complex. This week, let's look at how you can leverage these two techniques with Exchange Server.

First, let's define snapshots and clones. It's important to understand the difference between these two data duplication techniques. Simply put, a snapshot is a metadata mapping that points to a set of volume blocks that represent the data at the time the snapshot was created. A clone, on the other hand, is a complete copy of the data that has been created as an additional member in a mirror set and has been broken or split from the mirror. Both of these techniques can provide a rapid recovery mechanism for Exchange.

So, what are the caveats, and how do you best use these two techniques? First, you need to know that Microsoft doesn't support the use of snapshots or clones as a recovery measure for Exchange. However, the company released support guidelines for customers who want to use these technologies (available from various vendors, such as Compaq and EMC). Microsoft articles Q221756 and Q237767 discuss the support boundaries and procedures for using snapshots and clones with Exchange. Note that these articles apply to Exchange 5.5, and I don't know whether Microsoft will update them for Exchange 2000.

Because Microsoft doesn't really support your use of snapshots and clones, you must rely on your vendor for a solid solution and good support. Don't get me wrong; if you decide to deploy these technologies, I think they have a place—as a rapid recovery measure for lost or corrupt Exchange databases. But you should use them only to complement online Exchange backups—NOT to replace them. Administrators who deploy either technology as a replacement for supported online backups might find themselves without a job (after the CIO's mailbox is lost).

On the positive side, using snapshots or clones as a complementary recovery measure for Exchange has many benefits. Suppose your database becomes corrupted. A restore from tape could take hours, whereas a restore from a clone might take minutes. Or suppose you need to restore an individual mailbox. Mounting a cloned copy of your Exchange database to a recovery server can save a great deal of time over the traditional restore from tape and then to the personal store (PST) approach. Snapshots and clones are powerful Exchange tools, but use them with care and with the understanding of where Microsoft's support boundaries are.

In the upcoming Windows XP OS, Microsoft will add Volume Snapshot Services (VSS), which future Exchange 2000 updates will support, so Exchange 2000 on Windows XP Server should support snapshot and clone technology for restore operations; however, many of the specifics are still up in the air, and hardware vendors will have to provide support for VSS as well. So check out snapshots and clones as data recovery options; they might prove useful to you in your quest for more reliable Exchange servers.

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