FAQ:Should I use a holdingdisk when the final destination of the backup is a virtual tape?
From The Open Source Backup Wiki (Amanda, MySQL Backup, BackupPC)
Usually the answer is "yes"!
Holding disks and vtapes serve a complete different purpose: holding disk allows different clients to backup simultaneously and the server collects the incomplete images in the holding disk. When a backup image is complete, the server then adds it to the tape queue. The taper can choose the best file, depending on the criteria you set in "taperalgo". Especially "taperalgo largestfit" is helpful fitting as much as possible on a vtape. (e.g. when you want to burn the backups to a DVD later).
When you do not specify a holding disk, then only one DLE can be dumped at once. No parallelism is possible. And taper cannot use any of its algorithms either. This is true even when using vtapes.
In the case where you have only one client (e.g. your home PC), then avoiding a holding disk could indeed be a reasonable decision. But if your vtapes live on a slow USB-1 connected device, then a holding disk might still be faster. In case there are problems with your vtapes (e.g. the external disk with vtapes is full, or got disconnected), then Amanda can still fall back to degraded mode, and dump only incrementals to the holding disk.
Based on text by Stefan G. Weichinger, November - December, 2003, with updates in April, 2005.
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