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BACKUP.UNET automatically manages your backup volumes. It labels backup tapes and tells you which storage tape is ready to re use for a backup and checks to see that the correct tape is mounted.
A backup volume is a single volume, for example an 8mm tape, that has been labelled by BACKUP.UNET. This type of label is created electronically at the beginning of the tape, and identifies and describes the data recorded on the tape. The volume's label protects it from being inadvertently overwritten by BACKUP.UNET.
Backup volumes of the same type and size are grouped into media pools. (For information on media pools see Chapter 10, Managing Media Pools.) BACKUP.UNET uses media type files to classify different types and sizes of volumes.
Media type files are used by BACKUP.UNET when labeling backup volumes. There is a media type file for each type of backup media. The media type files are located in the '$NBK/nbk-TYPES' directory.
Each type file contains the capacity [-s size] and location
[-l location] arguments for each media type used by BACKUP.UNET. For example, the 'tk50' type file contains the following arguments:
-s 170000
-l "Computer Room"
where -s indicates the size of the computed volume capacity (170,000 blocks) and -l is the location ("Computer Room").
BACKUP.UNET supplies pre defined media type files; however, you can modify the media type files and define new media type files for additional types that are not included in the default types.
For detailed information on the default media type files in the '$NBK/nbk-TYPES' directory, see Chapter 15, BACKUP.UNET Home Directory Reference.
Adding a tape volume to a media pool creates an electronic label on the tape and adds an entry in the pool's volume table.
Before you can add a media volume to a pool, check for an existing media type file. If the appropriate media type file does not exist, you will need to compute the capacity of a mounted volume, and create a new media types file.
Computing volume capacity determines the recommended storage capacity (usually 90% of total available capacity) for any type of media supported by BACKUP.UNET.
It computes the recommended storage capacity, in data blocks, of a currently mounted volume by writing binary zeros to the volume. Be sure that the contents of the mounted volume can be erased, or that the volume is empty. BACKUP.UNET will not overwrite a current BACKUP.UNET volume and returns an error message for this condition.
A BACKUP.UNET volume label contains blocking factor, block size, and byte swapping information.
The data on backup volumes is written and read in records. The record size is computed by multiplying the blocking factor by the block size. The default block size is 512 bytes.
When backup media pools are created, a default blocking factor and block size are set. Operations involving the volumes in a pool automatically use the default blocking set for that pool.
When you add a new volume to a pool or compute the volume's capacity, the default blocking factor for that pool is used. The capacity of a volume is always reported as a multiple of 512 bytes per block.
The block size is specified in increments of 512 bytes.
If you choose a block size higher than that specified for the selected pool, you may be unable to use this volume for backups unless you also change the size of the block size specified in the pool description.
The Byte swap option allows you to specify whether or not bytes are to be swapped when writing to a volume. If you are creating a volume on one host that may be used on another host with a different byte order, you may want to specify that bytes are to be swapped. The default is no.