HP D2D100 Backup System - Manual (Page 98)

Compression
The HP D2D Backup System does not apply hardware compression to backed-up data; HP StorageWorks
tape drives do apply hardware compression. Therefore, 200 GB of data on the HP D2D Backup System
will require less space on a physical 200 GB tape cartridge. If moving data to a physical cartridge for
offsite storage, there is no need for concern if the data seems to utilize less space.
The capacity quoted for physical media assumes that the tape drive will be able to compress your data
and reduce the file sizes by 50%, but this can be achieved only if your data is not already compressed.
For example, JPEG pictures, movies, music or ZIP files, are already in compressed formats: spreadsheets,
text files, high-resolution graphics are not. Most users have a mixture of compressed and uncompressed
data and seldom achieve 50% compression.
Some users may choose to enable software compression from their backup application to increase
the storage space available to the backup application, but this can make backup jobs slower to run.
However, once software compression has been applied, the data is not further compressed when it is
exported to a physical tape directly attached to the HP D2D Backup System. This ensures that the user
knows exactly how much physical cartridge space is required for export or copy.
98 Export, import and copy using an attached tape drive