Slow down those streaming tape drives

Storage Networking World - If you have more than one state-of-the-art tape drive behind your LAN-based backup server, chances are you have too many. That's right. If you've got a LAN-based backup server, and behind it is more than one tape drive capable of 45 MB to 50 MB, then you should probably rethink your design. Let me explain.

It has to do with the nature of streaming tape drives. To fully realize their advertised throughput, you must supply these tape drives with a constant stream of data equivalent to their native throughput times, no matter what compression ratio you're getting. Sending data at a slower rate will actually cause it to write more slowly than the speed of the stream of data you're sending. The result in many backup systems is an array of tape drives that are writing at a fraction of their advertised speed. This has caused many customers to question the validity of the drives' advertised rates, when they should be looking at their design.

How to figure it out

When addressing this issue, there are three key numbers to consider: the drive's advertised native throughput, its capacity and your data's compression ratio. The Web site http://www.storagemountain.com/hardware-drives.html lists all popular drive types and their advertised native throughput rates. To determine your compression ratio, use your backup software product to determine how much data is being stored on tapes that it has marked "full." Divide the average size of your full tapes by the native capacity of the drive (also available at www.storagemountain.com, and you've got your compression ratio.

For example, if you've got a 9940B tape drive, its native throughput is 30 MB. If you're storing an average of 300 GB on its 200 GB tapes, then your compression ratio is 1.5:1. If you multiply 30 MB by a compression ratio of 1.5, you get a target throughput rate of 45 MB. If designed properly, your backup system should be capable of reaching this throughput rate on that tape drive in your environment.

Most streaming tape drives have two speeds: 0 MB (stop) and the target throughput rate. (See related story below "Variable Speed Tape Drives" for the exceptions to this rule.) Consider the 9940B above. It can only write at 45 MB in an environment with a 1.5:1 compression ratio. As soon as you drop below 45 MB, the buffer that is being used to hold data before it goes to tape will be emptied before it is filled up again.

When that happens, the tape drive does not slow down. It keeps writing at 45 MB until the buffer is empty. Then it stops, rewinds and waits for the buffer to be full again before repositioning and starting up another 45 MB stream. If you're supplying less than 45 MB, it will again empty the buffer, stop, rewind and reposition.