FAQ:What's the difference between a device and a changer? Between tapedev and tpchanger?

This article answers a FAQ (frequently asked question).

Devices and Changers

The best way to distinguish a device from a changer is by analogy to real, physical tapes. If you want to use more than one tape in your Amanda configuration, you need:

* some tapes
* a drive
* something to swap tapes (either a robot or a junior sysadmin)

Amanda's terms for these are, respectively:

* volumes
* devices
* changers

When Amanda needs a tape, it asks the changer for a device, and the changer does whatever's necessary - moving a tape into a drive or adjusting some symlinks for vtapes - and returns an appropriate device.

The model gets a little less obvious for things like VFS devices and Amazon S3, but it's still there! For S3 backups, for example, each volume has its own device, e.g.,

Volume

DailySet-004

Device

s3:mybucket/TAPE-004

Changer

chg-multi:s3:mybucket/TAPE-{001,002,003,004,005,006,007}

Configuration (tapedev and tpchanger)

New configurations only need to specify a changer. That changer will produce devices. In order to ensure backward compatibility, that changer can be specified in either tapedev or tpchanger. So, for example

 tpchanger "chg-disk:/path/to/vtapes"

or

 tapedev "chg-robot:/dev/sg2"

or

 tpchanger "my-changer-config"
 define changer "my-changer-config" {
   ...
 }

If you are looking at an older configuration, you may find both set along with some other options, e.g.,

 tpchanger "chg-zd-mtx"
 tapedev "/dev/nst0"
 changerdev "/dev/sg0"
 changerfile "/etc/amanda/myconfig/changer.conf"

If you have such a configuration, it should still work. If you want to upgrade it to use functionality available in newer versions of Amanda, consult the mailing list or #amanda.

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