The tape devices in AIX are designated as rmt devices. The rmt are specifically used for backups. During the study of AIX tape backups, lots of confusion arises in the terminology. Before discussing in detail about rmt tape types I would like to discuss these six confusing terms used in context to tape backups and restores:
Two more terms I’ll be using just for making my explanation easy. These are not standard terms.Â I am just using these to make my work easy for explanation purpose:
The term “Zero” will designate the start of tape device.
The term “End” will designate the end of the tape device.
Let’s suppose this tape is being used to store multiple backups. And let’s further assume that the tape is in its Zero position. We say it’s rewinded now. Let’s say we want to restore some backup from the tape which is 6th (just for an example), in sequence. Being a sequential device, a tape does not allow us to access backup number 6 directly. To reach 6th backup number we have to pass through previous 5 backups. That’s enough for the start, now it will be easier to understand retension and rewind.
Â On-Open means the start of some backup/restore operation.
On-Close means the finishing of some backup/restore operation.
Â To reach the 6th backup, the device pointer needs to move along the tape. To make this possible the tape opens up. This opening up of tape is called retension. Retension thus refers to opening of tape.
If you are at 6th backup in the tape. Now, you want to reach back at the start(Zero) of the tape. For this you need to close the tape. This is called rewinding of tape. It’s exactly the opposite of retension.
Before we start backup/restore operation on tape, the tape needs to be opened. Retension-on-open means that when tape is opened to backup/restore some file, the tape will be rewinded to Zero. So, device pointer will be at the Zero of the tape before the backup/restore begins.
If backup/restore operation on tape is finished, the tape will be rewinded back to its Zero point. Retension-on-close means, tape will be closed after the backup/restore is finished. So, device pointer will be at the Zero of tape after the backup/restore is finished.
Let’s summarize what we’ve learnt till now:
|Retension||Open the tape device for backup/restore operation.|
|Rewind||Close the tape device after backup/restore operation.|
|Retension-on-start||Before backup/restore operation starts, reach the Zero of tape.|
|Rewind-on-close||After the backup/restore operation is finished, reach the Zero of tape.|