How faster tape drives can slow down your backups


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With so many disk-to-disk backup options available, it just doesn't make sense to back up directly to tape over the network.

A LAN-based backup system can't deliver the throughput required to stop a modern tape drive from "shoe-shining." Simply put, you can't back up via a LAN directly to today's high-speed tape drives without an interruption in the data stream; this requires the tape to be repositioned on the drive's heads, which causes a "stop-start" or "shoe-shine" motion on the tape device that slows performance.

To record a high-quality signal to tape, the recording head must be moved across the media very quickly. This is why modern tape drives have a minimum speed; if they went slower than their rated minimum, they wouldn't record a good quality signal and would lose data. This is an unchangeable fact about tape drives: They have a minimum usable speed.

That's why all tape drives designed for large-scale backups have two speeds: stop and very fast. This also applies to variable-speed tape drives. If you have a 50MB/sec tape drive and you send it a data stream at 25MB/sec, it doesn't write at 25MB/sec. Instead, it fills up a buffer and writes short bursts at 50MB/sec; it then stops, rewinds and prepares itself to write another short burst at 50MB/sec. The process of stopping, rewinding and getting back up to speed is called repositioning or backhitching. Each reposition/backhitch can take as long as a few seconds. If you backhitch

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a lot, it's called shoe-shining because the tape activity mimics the movement of a cloth being used to shine shoes.

The further you operate from the drive's target throughput rate, the less time you spend writing data. Send a 50MB/sec tape drive a 40MB/sec data stream and it will actually write at 35MB/sec because it's spending at least 20% of its time shoe-shining. Send it data at 30MB/sec and it will write at 20MB/sec; send data to the drive at 20MB/sec and it will write at 10MB/sec; and if you send it at 10MB/sec, you may see the tape drive write at a rate of less than 1MB/sec.

This was first published in September 2006

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