Sun Microsystems Virtual Tape Library manual Sun recommends capacity on demand for most users

Managing virtual libraries

6.When the Enter virtual tape properties dialog appears, check the Tape Capacity On Demand check box (E below).

Sun recommends capacity on demand for most users.

E

F

G

7.If you are not using tape caching, use the Maximum Capacity spinner control (F above) to set the maximum size of your virtual tapes to 50‐60 GB or to the average size of your backup jobs (whichever is larger).

This approach minimizes wasted space in the critical disk cache, where the first copy of each backup job is stored. It maximizes the number of cartridges available for backup jobs at any given time. Your copy/vault software can then consolidate these smaller volumes onto full‐sized physical tape cartridges outside the backup window.

8.If you are using VTL data compression with tape caching, use the Maximum Capacity spinner control (F above) to reduce the Maximum Capacity to 85‐90% of the encompassed capacity of the selected media.

In the example above, you would reduce maximum capacity to 42‐43 GB when using compression.

When you enable tape caching, a single virtual tape has two images, one in the disk cache and one in the tape cache. Both images must be logically identical. But when you use compression with tape caching, you cannot be sure that the disk and tape images will be identical. Different compression algorithms can produce slightly different compression ratios. So, unless you allow a 10‐15% margin of safety, you might find that a compressed data set that fills a tape image on disk will not fit on a physical cartridge.

9.If you are using tape caching, leave the Maximum Capacity control set to the maximum capacity of the physical media, unless you are also using VTL data compression.

The disk and physical‐tape images of a virtual tape must be logically identical when you enable tape caching. They cannot be different sizes.

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