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November 28, 2006

Tape isn't dead, but...

This article from The Register emphasizes that, even though high-density disk drives have come down in price dramatically, tape is still important in an overall backup strategy. This is all true; tape isn't going away for a long time, but the author misses the point on the specifics.

In comparing the two, he says:

Disk's main advantage is speed, of course. That's not so much the speed at which you can write data to it - modern tape drives can stream data in and out pretty darn fast - but the speed of recovering files from it.

That's because disk is a random-access technology, so it takes the same length of time to get to data anywhere on the disk, whereas tape is sequential-access, and you must spool through the whole cartridge to get to data that's written towards the end.

Yes, it's slow and annoying to get a file that's way back on the tape cartridge because it's sequential access, but that's not the issue at the enterprise level. Even with high-end tape libraries (the sort of subsystem that's the size of your refrigerator and can hold 96+ tapes at a time), it's going to take half a day to back up a 1TB+ filesystem. That means it's going to take half a day to restore that filesystem, giving you Recovery Point Objective (RPO) and Recovery Time Objective (RTO) times of 12+ hours. In a competitive environment where losing minutes worth of data can be costly to an enterprise, tape becomes what EMC calls an "unacceptable backup strategy."

The solution is disk. Replicating large-scale volumes to disk reduces RPO/RTO times to a fraction of those of a tape-based solution.

Still, you can't just put a backup disk subsystem on a truck and send it over to the off-site storage facility like you do a box of tapes. That's why tape will be around for a good while still. The cartridges are inexpensive and easily transportable.

Posted by Edward J. Branley at November 28, 2006 1:09 PM

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