How faster tape drives can slow down your backups


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The perfect storm
We've now established that the slowest these tape drives can go is between 60MB/sec and 75MB/sec--but they should really be going between 158MB/sec and 180MB/sec. Let's compare these numbers to the ability of a backup server to receive that amount of data via a Gigabit Ethernet network card. Based on what I've seen, most backup servers tend to hover around 50MB/sec to 60MB/sec. Servers don't tend to get much faster with multiple network interface cards (NICs) because the problem is the number of interrupts Ethernet requires. Yes, there are exceptions to this, but this is what I usually see. Chances are it's what you'll see if you test your backup server.

The fastest tape drive wants a stream of approximately 180MB/sec, but will operate efficiently at a rate as slow as 90MB/sec. If the backup server is linked to that tape drive with a Gigabit Ethernet connection, it probably won't receive data much faster than 60MB/sec.

It's important to carefully examine the number of tape drives that simultaneously share the same network connection. The speed of the tape drive on the back end needs to match the speed of the pipe on the front end. Use multiple tape drives and you're asking all of those drives to share the 50MB/sec to 60MB/sec pipe, which means each drive gets a fraction of what it needs to stream.

Even if you had an Ethernet NIC capable of receiving 180MB/sec, would you want to stream a tape that fast? Remember that we're still

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backing up fragmented file systems, and the throughput of each individual network backup is often going to be less than 10MB/sec, so you're going to need 20 or more simultaneous backup streams multiplexed together to stream that drive. Can you imagine how slow your restore performance would be if you were reading only one stream from a backup tape with 19 other jobs multiplexed to it?

This means that it's impossible to follow the best practice and back up only to tape. If you put a single 180MB/sec tape drive behind a 50MB/sec to 60MB/sec pipe, you're just asking for trouble. It also means that if you simply improve the speed of your tape drives, your backups will take longer to complete.

That's why disk-to-disk-to-tape makes so much sense. If you back up to disk across the network, speed isn't an issue. It's no problem to back up highly fragmented file systems at a few megabytes per second; once the backup completes, take the fragmented file system and put it on a contiguous section of disk on the backup server. The backup server should then be able to copy that backup to tape much faster than it would have backed it up to tape across the network.

The great thing is that you can do all of this without using multiplexing. The disk is moving data from local, contiguous sections of disk directly to tape. Think about what that does for your restore speeds. Disk-based backup is now a necessity. At this point, it's impossible to properly design a backup system without it. You can still use tape--just don't back up directly to tape across the network.

This was first published in September 2006

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