TCOM: Dynamic Tape Compression

To ensure optimum data backup on LTO tapes, a minimum data transfer rate must be achieved so that the tapes runs continuously (“streaming mode”).

If the minimum data transfer rate is not achieved, the tape is slowed by the device, rewound a little, and then repositioned behind the last data written. This procedure (start/stop mode) is not only time-consuming, but also reduces the service life of the tapes.

The required minimum data transfer rate can be achieved using ARCHIVE and data on high-speed disks and “large” Raid systems (see the “Introduction to System Administration” [5 (Related publications)]). It is not achieved with low-speed disks.

“In between” there is an area in which the minimum data transfer rate is achieved if compression on the device is switched off. The tape capacity is correspondingly lower when compression is switched off.

Selecting the compression using TCOM

Compression on the LTO device is switched on and off using the TCOM (Dynamic Tape Compression) function. By default, compression (also without IORM) is switched on.

TCOM also enables compression to be switched on and off dynamically, i.e. in accordance with the data transfer rate. In this case

• compression is switched off when the data transfer rate without compression is above the minimum value required for streaming mode, but not if compression is used.

• compression is switched on when the data transfer rate is above the minimum value required for streaming mode even if compression is used.

In the case of /390 servers, the device notifies TCOM directly of the data volume transferred from the server to the device and from the device to tape. In the case of x86 servers, only the data volume transferred from the server to the device is known; the data volume transferred from the device to tape is calculated from the compression factor specified.

 

Controlling compression for LTO devices

TCOM_SET controls whether compression is switched on or off for LTO devices. The setting applies for all LTO devices in the home system. The current setting can also be queried.

TCOM_SET                                                                                                                                             

TCOM_SET=ON

TCOM_SET=OFF

TCOM_SET=DYN

TCOM_SET?

ON	Switches compression on for LTO devices (default value). This setting ensures that optimum use is made of the tape capacity. However, when the data transfer rate is low, the tape often switches to start/stop mode.
OFF	Switches compression off for LTO devices. When the data transfer rate is low, the number of start/stop events can be reduced; the tape capacity decreases in accordance with the compression rate for the data.
DYN	Switches compression on or off for LTO devices in accordance with the data transfer rate. This setting combines the benefits of optimum tape capacity when the data transfer rate is high and a low number of start/stop operations when the data transfer rate is low.
?	The current setting is queried.

Defining the compression factor

On x86 servers TCOM_FACTOR defines the compression factor for data which is to be saved for TCOM_SET=DYN. The current value can be queried.

TCOM_FACTOR                                                                                                                                     

TCOM_FACTOR=n.m

TCOM_FACTOR?

n.m	Compression factor on x86 servers and when TCOM_SET=DYN. Value range: 1.0 <= n.m <= 9.9 Default value: 2.0
?	The current setting is queried.