Compression decreases the amount of data sent over the networks and the space occupied by the same data in storage. Compression saves throughput time but requires more memory and CPU cycles. Typically, a system with a slow processor will not benefit from compression, especially if it is connected to the server through a high-speed link.

Compression of data was already offered as a feature in BackHome!. BackHome!/TSM (formerly BackHome!/ADSM), uses the compression provided by the Tivoli Storage Manager API. If you think that you could benefit from compression, you need to have a good understanding of the different settings involved, as well as their effects.

This technical note reviews the different issues concerning compression in BackHome!/TSM.

1) Administrator setting

The Tivoli Storage Manager administrator may determine the compression of all objects sent by the client using the following setting in the Tivoli Storage Manager Administrative client: COMPression=[CLIENT|YES|NO]

a)       Client determined (CLIENT) - The Tivoli Storage Manager client (BackHome!) will specify if the object was compressed or not.

b)       Always (YES) - The server will force the Tivoli Storage Manager API to compress the object. The compression setting specified by the client will be ignored.

c)       Never (NO) - The server will not allow the Tivoli Storage Manager API to compress the object. The compression setting specified by the client will be ignored.

Or when defining a node using the Tivoli Storage Manager Administrator command:

REGister NODE nodename

2) The client system file. (DSMSYS)

The compression setting inside the client option file is only effective when it is allowed by the administration setting (see 1-a).

a) COMPRESSION NO     – Files will not be compressed by the API before being sent to the server.

b) COMPRESSION YES    – Files will be compressed by the API before being sent to the server.

3) The client options file. (DSMOPT)

For this keyword to take effect, compression must be enabled in DSMSYS.

a)       COMPRESSALWAYS YES. Compressed objects that end up being bigger than the original object are still sent to the server.

b)       COMPRESSALWAYS NO. Compressed objects that are bigger than the original objects are discarded and the original object is sent.

4) The BackHome!/TSM setting

BackHome!/TSM (BackHome!/ADSM) uses two keywords in the BCOMCONF file and the request respectively to turn on or turn off the compression. The section ADSM-REQUESTOR allows the administrator to specify a compression setting with the keyword

ENCOMP [NO|YES|ALWAYS]

a)       NONo compression regardless of request setting.

b)       YESObject will be compressed if request setting allows it.

a)       ALWAYSObject will always be compressed regardless of request setting allows it.

The administrator can also specify the compression setting on a request by request basis. The syntax is

COMPRESSION [NO|YES].

In BackHome!/ADSM V4, an object will be compressed based on the configuration setting of the request and of the requester. The compression setting in effect for the node on the Tivoli Storage Manager server and that coded in DSMSYS override the setting of the requester and request. The following table shows the effect of the different settings.

In BackHome!/TSM V5, the compression settings have been simplified. The ENCOMP parameter has been dropped for ADSM requestor in the BCOMCONF file and if coded, will be ignored. Only the request compression field will be used to determine if an object will be compressed or not. Of course COMPRESSION YES must be coded in the DSMSYS file.