Quantum Audio drive DDS-4 - Manual (Page 23)

Installing internal tape drives Reviewing Drive Default Settings
23
Data Compression
The internal tape drive provides two switches for controlling data compression.
Switch 1 determines whether hardware data compression is enabled or disabled. By default, switch
1 is set to ON, enabling hardware data compression. To disable hardware data compression, set
switch 1 to the OFF position.
Switch 2 controls whether SCSI commands are recognized for enabling or disabling hardware data
compression. By default, switch 2 is set to ON, allowing SCSI commands to be used to enable or
disable hardware data compression. To prevent SCSI commands from enabling or disabling hard-
ware data compression, set switch 2 to the OFF position.
When data compression is enabled, the drive compresses all data written to tape and decompresses
all compressed data read from tape. The degree of compression varies due to the type of data being
processed.
• Data with a high degree of redundancy, such as structured database files or graphics files,
are compressed most efficiently, often at a ratio of 2:1 or more.
• Data with little redundancy, such as executable programs, are compressed the least.
Media Recognition System (DDS-4 Drives Only)
Using non-DDS media may appear to give satisfactory results, but the inferior specifications of such
media can cause data-integrity problems. To avoid these problems, the internal tape drive provides
a media-recognition system (MRS) feature that determines whether tape cartridges conform to the
DDS tape standard.
By default, switch 3 is set to ON, enabling MRS. With this setting, the drive:
• Reads from and writes to MRS media.
• Reads from but does not write to non-MRS media.
To disable MRS, set switch 3 to the OFF position. With this setting, the drive reads from and writes to
MRS and non-MRS media.
NOTE: Switch 3 is reserved on internal DAT 72 drives.
Settings: Switch 1 ON = Enable hardware data compression (default)
Switch 1 OFF = Disable hardware data compression
Switch 2 OFF = Recognize SCSI data compression commands (default)
Switch 2 ON = Ignore SCSI data compression commands
Settings: Switch 3 ON = Enable MRS (default)
Switch 3 OFF = Disable MRS