Tapes Are Not Filled to Capacity

You might encounter situations where Backup does not fill tapes to capacity. For example, a tape with an advertised capacity of 4000 MB can be marked full by Backup after only 3000 MB of data have been written to it.

To enable Backup to use the tape capacity to its fullest, select the highest density device driver appropriate for your device. When a tape is labeled, Backup writes to it at the highest density supported by your device.

There are several reasons for situations in which Backup appears to fill tapes prematurely:

• Write errors occur during a backup.

  Most tape drives try to read after a write operation to verify that the tape was written correctly, and retry if it was not. A write error indicates either end of tape or a read error. At any tape error, Backup marks the tape full.

  To prevent tape write errors, clean your tape drive regularly and use only data-quality tapes. If cleaning the drive does not seem to help, make sure that the device driver is properly configured, any necessary switch settings on the tape drive are set to the manufacturer's specifications, all cabling is secure, and other potential SCSI problems have been addressed.

• Backup filemarks take up space on the tape.

  Backup periodically writes filemarks to facilitate rapid recovery of data. These filemarks consume varying amounts of tape depending on the type of tape drive on some drives, filemarks can consume several MB. The number of filemarks Backup writes to tape is a function of how many save sets are on the tape. Many small save sets require more filemarks than a few larger ones.

• Tape capacities vary from tape to tape.

  Tape capacities are not constant from tape to tape. Two apparently identical tapes from the same vendor can vary significantly in capacities. This can cause problems if you copy one very full tape to another, especially if the destination tape holds less data than the source tape.

• Data compression affects the tape capacity.

  If you use compression on your tape drive, you cannot predict the effect on tape capacities. A compressing drive can provide twice the capacity of a non-compressing drive. It could be far less or far more, depending on the kind of data being backed up. For example, if a noncompressing drive writes 2 GB of data to a specific tape, the compressing drive could write 10 GB, 2 GB, 5 GB, or some other unpredictable amount of data.

• Length of tape.

  Be sure to verify tape lengths. A 120-meter DAT tape holds more data than a 90-meter DAT tape, and without examining the printed information on the tape cassette carefully, the two tapes can appear identical.

  For Solaris, if your tape devices are not directly supported by Sun Microsystems, you will need to recreate your entries in the st.conf file. If you need assistance with this, contact Sun Technical Support.