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Tape libraries and the hidden speeds

Posted by Preston on 2009-02-25

There was recently a discussion on the NetWorker mailing list that was sparked by someone asking what sort of LTO-4 library would be recommended by the community.

This led to some very interesting and useful feedback to the person posing the question. A lot of people had feedback about different libraries they’d used – both good and bad – and questions to ask, such as slot count and CAP/Mail slot size. I felt it was also important to weigh in on media movement speed, as I think this is often something which is disregarded when evaluating libraries – even though it often can play a factor in backup throughput, usability and perceived performance.

Rather than summarise myself, here’s what I had to say on the topic then:

Too often people worry about the speed and capacity of the media, and forget about the incidental factors, such as robotic movement times and even load/seek time on the media. These can play an important factor in backup and  – more importantly, really – recovery schedules. When it comes to measuring backup performance, the sequence of “returning to slot, picking next tape, placing in drive” can actually start to make a significant impact on what I refer to as your overnight “backup bandwidth”. If it takes say, 70 seconds for one library to do it and your drives write at 160MB/s, then that’s a 10GB interruption to your backups. If another library can do the same thing in 30 seconds, that’s just a 4.7GB interruption to your backups. (I’m deliberately excluding load/unload times of the media, because in a realistic comparison it would be the same drives in both libraries…)  Repeat that say, 30 times a night, and suddenly you’re deciding whether you can afford to lose 300GB in backup time a night or 141GB in backup time a night. For bigger sites, these numbers can actually become very important.

If you are considering a new tape library, be sure to ask about not only the “easy” numbers, such as how much it costs, how much maintenance costs, and how fast the drives read/write, but also the more challenging numbers – how much time it takes to move media around.


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