The DEFINE DEVCLASS command has an optional ESTCAPACITY parameter that indicates the estimated capacity for sequential volumes associated with the device class. If the ESTCAPACITY parameter is not specified, ADSM uses a default value based on the DEVTYPE parameter of the device class.
If you specify an estimated capacity that exceeds the actual capacity of the volume in the device class, ADSM updates the estimated capacity of the volume when the volume becomes full. When ADSM reaches the end of the volume, it updates the capacity for the amount that is written to the volume.
You can either accept the default estimated capacity for a given device class, or explicitly specify an estimated capacity. An accurate estimated capacity value is not required, but is useful. ADSM uses the estimated capacity of volumes to determine the estimated capacity of a storage pool, and the estimated percent utilized. You may want to change the estimated capacity if:
Client files can be compressed to decrease the amount of data sent over networks and the space occupied by the data in ADSM storage. With ADSM, files can be compressed by the ADSM client before the data is sent to the ADSM server, or by the device where the file is finally stored.
Use either client compression or device compression, but not both.
The following table summarizes the advantages and disadvantages of each type
of compression.
| Type of Compression | Advantages | Disadvantages |
|---|---|---|
| ADSM client compression | Reduced load on the network | Higher CPU usage by the client
Longer elapsed time for client operations such as backup |
| Drive compression | Amount of compression can be better than ADSM client compression on some drives | Files that have already been compressed by the ADSM client can become larger |
Either type of compression can affect tape drive performance, because compression affects data rate. When the rate of data going to a tape drive is slower than the drive can write, the drive starts and stops while data is written, meaning relatively poorer performance. When the rate of data is fast enough, the tape drive can reach streaming mode, meaning better performance. If tape drive performance is more important than the space savings that compression can mean, you may want to perform timed test backups using different approaches to determine what is best for your system.
Drive compression is specified with the FORMAT parameter for the drive's device class, and the hardware device must be able to support the compression format. For information about how to set up compression on the client, see "User Registration of Client Nodes" and "Administrator Registration of Client Nodes".
How ADSM views the capacity of the volume where the data is stored depends on whether files are compressed by the ADSM client or by the storage device. It may wrongly appear that you are not getting the full use of the capacity of your tapes, for the following reasons:
Figure 14 compares what ADSM sees as the amount of data stored on tape when compression is done by the device and by the client. For this example, the tape has a physical capacity of 1.2GB; however, the manufacturer reports the capacity of the tape as 2.4GB by assuming the device compresses the data by a factor of two.
Suppose a client backs up a 2.4GB file:
In both cases, ADSM considers the volume to be full. However, ADSM considers the capacity of the volume in the two cases to be different: 2.4GB when the drive compresses the file, and 1.2GB when the client compresses the file. Use the QUERY VOLUME command to see the capacity of volumes from ADSM's viewpoint. See "Monitoring the Use of Storage Pool Volumes".
Figure 14. Comparing Compression at the Client and Compression at the Device
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For how to set up compression on the client, see "User Registration of Client Nodes" and "Administrator Registration of Client Nodes".