Practical UNIX and Internet Security, 3rd Edition

Software for Backups

There are a number of software packages that allow you to perform backups. Some are vendor-specific, and others are quite commonly available. Each may have particular benefits in a particular environment. We’ll outline a few of the more common ones here, including a few that you might not otherwise consider. You should consult your local documentation to see if there are special programs available with your system.

Beware of Backing Up Files with Holes

Standard Unix files are direct-access files; in other words, you can specify an offset from the beginning of the file, and then read and write from that location. If you have ever had experience with older mainframe systems that only allowed files to be accessed sequentially, you know how important random access is for many things, including building random-access databases.

An interesting case occurs when a program references beyond the “end” of the file and then writes. What goes into the space between the old end-of-file and the data just now written? Zero-filled bytes would seem to be appropriate, as there is really nothing there.

Now consider that the span could be millions of bytes long, and there is really nothing there. If Unix were to allocate disk blocks for all that space, it could possibly exhaust the free space available. Instead, values are set internal to the inode and file data pointers so that only blocks needed to hold written data are allocated. The remaining span represents a hole that Unix ...