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Don't Let Back-Ups Back You Up

Backups are still necessary.

Backups are still necessary.

The Discipline

Since we have very few true back-up products that can do transaction level back-up, we have to maintain or re-start our processes for backing up data to catch logical and user errors.

We must maintain our log copies (or change accumulations, if they exist) and our image copies. We must have our roll-forward and roll-backward processes up to date. And all processes must be tested on a regular basis.

This most critical test is one that most shops miss. I hate to admit the number of times that I have not found out that a back-up was invalid until I needed to use it. The data I needed was lost or corrupted at the primary location, along with all my fancy disk-to-disk copies - and last night's back-up.

Back-Up Options

IBM and others have introduced many remote and local disk-to-disk back-up implementations. The IBM mainframe versions are Peer-to-Peer Remote Copy (PPRC), eXtended Remote Copy (XRC), and FlashCopy. Of course, there is always tape, for which IBM has also implemented remote mirroring.

PPRC allows for synchronous mirroring for any DASD I/O at the individual block level. As each I/O is processed, it is propagated to a remote site and the operation is not completed until the remote - and the local - I/O is done. Due to limitations such as speed of light, this is only feasible online for a few (less than 40) kilometers.

For longer distances, XRC is the preferred approach. This removes the delay from synchronizing the remote copy of the I/O, but introduces a problem of out-of-sync I/O operations. So, a copy of the data could be suspect in the event of a failure. All asynchronous operations suffer from this drawback.

Ted MacNeil is a capacity/performance analyst with more than 25 years in the IBM mainframe environment. Ted can be reached at tedmacneil@alumni.uwaterloo.ca.



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