Large File System Backup Tool

W. Curtis Preston

For this edition of lost+found, I thought I'd talk about a solution to a problem that plagues both large and small sites. Since the solution requires a script, this column will be a mix of backup and scripting information. I hope you find it useful.

Whether you've got a small site with one or two backup drives to back it up with, or a 10-TB filesystem, I think you might find yourself suffering from a problem that I have finally come up with a solution for. (Although, I admit that large sites probably suffer from this problem more than small sites.)

The Problem

Have you ever needed to back up a filesystem that was too large for one backup job? Perhaps it was an 8-GB filesystem, and you happen to have two 5-GB tape drives, but you don't know how to get the backup to start on one tape and move over to the other. Perhaps you've taken advantage of 32-bit operating systems, and have started using terabyte-sized filesystems. How do you back up a terabyte-sized mount point?

If you are like the shop described in the first example, the solution is not easy. Most backup programs (including dump, tar, and cpio) do not have any way to start a backup on one tape drive and then move it to another. They only understand the concept of swapping tapes, and even that is usually meant to be done by a human. If you had a stacker and were good with expect, you could probably write a script that would watch for the tape prompt, swap the tape, then notify the backup program. However, how do you do that with two tape drives? (I suppose a real hacker could monkey around with symbolic links to device files, but that would be really naughty.)