Media formats
NetBackup writes media in a format that allows the position to be verified before NetBackup appends new backups.
The following table shows the symbols that are used in the media format descriptions.
Table: Media format symbols
Symbol | Description |
|---|
MH | Media header (1024 bytes). |
* | Tape mark. |
BH | Backup header (1024 bytes). |
BH1 ... BHn | Backup headers (1024 bytes). One for each job that is part of the set of the jobs that are multiplexed. |
Image | Data from the backup. |
EH | Empty backup header, which is used for position validation. |
The following table provides more information about how the media formats are used in different situations.
Table: Media format descriptions
Format | Description |
|---|
Standard tape format | For all tape media except quarter-inch cartridge (QIC) and WORM, the format for the backups that are not multiplexed is as follows:
MH * BH Image * BH Image * BH Image * EH *
When a new backup image is added, the tape is positioned to the EH and the position is verified. The EH is overwritten by a BH and the backup proceeds. When complete, a new EH is written for future position validation.
When NetBackup encounters the end of media during a write operation, it terminates the tape with two tape marks and does not write an EH. |
QIC and WORM tape format | This format is used for quarter-inch cartridge (QIC) and WORM media. Unlike the standard tape format, NetBackup does not write empty backup headers (EH). The format is as follows:
MH * BH Image * BH Image * BH Image *
To append backup images to QIC media, NetBackup positions to the end of data (EOD) and then starts the next backup. |
Fragmented backup format | For fragmented backups, the media format is similar to the standard tape format. The difference is that NetBackup breaks the backup image into fragments of the size that are specified when the storage unit is configured.
The following is an example:
MH * BH Image (frag 1)* BH Image (frag 2)* BH Image (frag n) * EH *
Fragmentation is intended primarily for storing large backup images on a disk type storage unit.
For multiplexed backups, image fragmentation results in faster restores because NetBackup can advance to the specific fragment before it begins a search for the file.
If an error occurs in a backup, the entire backup is discarded and the backup restarts from the beginning. It does not restart from the fragment where the error occurred. Exception: checkpoint and restart backups resume from the last checkpoint fragment. |
Multiplexing format | The tape format for multiplexed backups is as follows:
MH * BH1 ... BHn Image ...
By default, the data image is in 64-kilobyte blocks. Each block also contains 512 bytes that are reserved for multiplexing control information and to identify the backup to which the block corresponds.
When a job ends or a new job is added to the multiplexing set, NetBackup writes a tape mark. NetBackup then starts multiplexing the revised set of jobs.
The following is an example:
MH * BH1 BH2 BH3 Image* BH2 BH3 Image* BH2 BH3 BH4 Image |
Spanning tape format | By default, NetBackup spans a backup image to another tape if it encounters the end of media during a backup. The format is the same as described for fragmented backups. The first fragment on the next tape begins with the buffer of data where the end of media occurred.
The following is the first tape format (NetBackup does not write an EH and terminates the tape with two tape marks):
MH * ... *BHn Image (frag 1) * *
The following is the second tape format:
MH * BHn Image (frag2)* ... * EH * |
