All You Wanted To Know About AIX Tape Device Backups-II

March 28th, 2009

Welcome to the second article of the series about rmt devices in AIX. In first article I discussed about the common but confusing terminology used for tape devices. I hope you found the article All You Wanted To Know About AIX Tape Device Backups-I, interesting and useful. In this second article I am gonna explain the other naming conventions for rmt tape devices.

Backup Data Density
The backup data density is the number of Bytes per inch of the magnetic tape. This is used to decide how much data can be stored on a tape device.

Naming of Tape Devices
The tape devices in AIX are named as rmtM.N. Two things here worth noting are:

M ranges from 0 to 255.
N ranges from 1 to 7.

So, typically tape devices will be named like:
rmt0, rmt0.1, rm0.2, rmt0.3, rmt0.4, rmt0.5, rmt0.6, rmt0.7 etc.
..through rmt1, rmt1.1, rmt1.2, rmt1.3, rmt1.4, rmt1.5, rmt1.6, rmt1.7 etc.
……up to
rmt255, rmt255.1, rmt255.2, rmt255.3, rmt255.4, rmt255.5, rmt255.6, rmt255.7 etc.
And so on…

Special Characteristics of Tape Devices
For our example, I’ll use rmt0 as device name. Now, I explain the meanings of all the characteristics of tape devices.

rmt0
If some backup/restore operation is to be started the tape is not retensioned to start. The backup/restore operation will start at the point where the device pointer is at present. But, the tape is rewinded to Zero position once the backup/restore operation is finished.

The, settings are:
Retension-on-start: No
Rewind-on-close: Yes

rmt0.1
If we want to start backup/restore operation, the backup/restore operation will start from the previous point where the pointer was, at the end of previous backup/restore operation. Similarly, the tape is not rewinded to start after the backup/restore operation is finished. This is the most common option used in tape backups.

The settings are:
Retension-on-start: No
Rewind-on-close: No

rmt0.2
If some backup/restore operation is to start, the tape is retensioned to the Zero point. So, if we want to start backup/restore operation, the backup/restore operation will start from the Zero point of tape. Similarly, the tape is rewinded to start after the backup/restore is finished. This is also common option used in tape backups.

The settings are:
Retension-on-start: Yes
Rewind-on-close: Yes


rmt0.3
If some backup/restore operation is to start, the tape is retensioned to start. So, if we want to start backup/restore operation, the backup/restore operation will start from the previous point where the pointer was after the previous backup/restore operation. The tape is not rewinded to start after the backup/restore is finished.

The settings are:
Retension-on-start: Yes
Rewind-on-close: No

rmt0.4 corresponds to rmt0. The only difference is that the backup data density is lower.
rmt0.5 corresponds to rmt0.1. The only difference is that the backup data density is lower.
rmt0.6 corresponds to rmt0.2. The only difference is that the backup data density is lower.
rmt0.7 corresponds to rmt0.3. The only difference is that the backup data density is lower.

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One Response to “All You Wanted To Know About AIX Tape Device Backups-II”

  1. DLT Tapes Says:

    good work Ganesh! easy to read and understand the critical points. thanks

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