Running Linux, 5th Edition - Matthias Kalle Dalheimer [474]
Creating one tar file per tape might be wasteful if the archive requires only a fraction of the capacity of the tape. To place more than one file on a tape, you must first prevent the tape from rewinding after each use, and you must have a way to position the tape to the next file marker, for both tar file creation and extraction.
The way to do this is to use the nonrewinding tape devices , which are named /dev/nqft0, /dev/nqft1, and so on for floppy-tape drivers, and /dev/nst0, /dev/nst1, and so on for SCSI tapes. When this device is used for reading or writing, the tape will not be rewound when the device is closed (that is, once tar has completed). You can then use tar again to add another archive to the tape. The two tar files on the tape won't have anything to do with each other. Of course, if you later overwrite the first tar file, you may overwrite the second file or leave an undesirable gap between the first and second files (which may be interpreted as garbage). In general, don't attempt to replace just one file on a tape that has multiple files on it.
Using the nonrewinding tape device, you can add as many files to the tape as space permits. To rewind the tape after use, use the mt command. mt is a general-purpose command that performs a number of functions with the tape drive. The mt command, although very useful and powerful, is also fairly complicated. There's a lot to keep track of to locate a particular record on the tape, and it's easy to get confused. If you're particularly motivated to use your tapes as efficiently as possible, read up on mt; the manpage is quite concise. We include a few examples here. The command:
mt /dev/nqft0 rewind
rewinds the tape in the first floppy-tape device.
Similarly, the command:
mt /dev/nqft0 reten
retensions the tape by winding it to the end and then rewinding it.
When reading files on a multiple-file tape, you must use the nonrewinding tape device with tar and the mt command to position the tape to the appropriate file.
For example, to skip to the next file on the tape, use the command:
mt /dev/nqft0 fsf 1
This skips over one file on the tape. Similarly, to skip over two files, use:
mt /dev/nqft0 fsf 2
or
mt device fsf 1
to move to the next file.
Be sure to use the appropriate nonrewinding tape device with mt. Note that this command does not move to "file number two" on the tape; it skips over the next two files based on the current tape position. Just use mt to rewind the tape if you're not sure where the tape is currently positioned. You can also skip back; see the mt(1) manual page for a complete list of options.
Backing up to CD-R
You can back up your files to recordable CD perhaps more easily than to tape. Blank CDs are very inexpensive, widely available, readable on just about any system, and much easier to transport and store than tapes. In this section, we show you the basics of writing backups to CD-R, as well as a couple of tricks. Almost all of the techniques that work for CD-Rs work equally well for the various flavors of recordable DVDs that are available.
By far the most common way to write data to a CD is to create a CD image file on your hard disk, then burn that to CD. This is easy to do, but has one slight disadvantage: you need at least 650 or 700 MB of free disk space to create a full-sized CD image. On modern systems that generally shouldn't be a problem.
CD-ROMs use the ISO 9660 filesystem standard, which can be mounted and read on just about any operating system in common use today. The program mkisofs is a full-featured and robust tool for creating such filesystems, which can be used in a number of ways, including