How faster tape drives can slow down your backups


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Variable-speed tape drives
You may be wondering about variable-speed tape drives. You've likely heard that some tape drives, such as IBM Corp.'s System Storage T1120 (104MB/sec) and Sun Microsystems Inc.'s StorageTek T10000 (120MB/sec), have the ability to slow down to adjust to incoming data rates. You may even have heard from these vendors that this feature eliminates shoe-shining. While these drives help keep up with slower incoming data rates, they can't change the basic rule that tape drives have a minimum speed they must use to get a good signal-to-noise ratio. This means they can slow down, but they can't slow way down. IBM's T1120 can slow to a native speed of 40MB/sec, while the StorageTek T10000 can slow to a native speed of 50MB/sec.

It's time for some math. Let's consider the T1120 the fastest of these drives, with a native throughput of 104MB/sec. Now let's throw some compression into the mix. Remember that in addition to increasing your capacity, tape drive compression also increases your effective throughput. Based on what I've seen in hundreds of companies, I'm comfortable with using 1.5:1 as an average compression ratio in open-systems environments. If we use 1.5:1, the T1120 is actually a 180MB/sec tape drive. If we allow it to slow to 50% of that, this tape drive can write as slowly as 90MB/sec without shoe-shining. But if you send it something at a rate of less than 90MB/sec, it will backhitch. If you send it something at a rate significantly

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less than 90MB/sec, it will shoe-shine. The same logic applies to other variable-speed drives.

This was first published in September 2006

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