Understanding Volume Usage, Capacity, and Compression

DFSMSrmm tracks information about what you write onto tape volumes, including record length, block size and number of blocks. DFSMSrmm also tracks information about the data written to the tape volume, such as the compressed size and physical size of the file and can calculate the compression ratio. In addition, information is recorded about the media capacity, percentage used, physical space used, and the overall compression ratio.

Basic information about the media can be defined by command, but, when a volume is used for output, DFSMSrmm records the media type, recording format, capacity and percentage used. Normally this information is provided by the tape drive when the drive is an IBM® 3590, or IBM TS11x0 or later drive. For older tape devices such as 3490, and also for virtual tape emulating 3490, this information is not available and is derived from media information hard-coded in DFSMSrmm. For non-IBM media, and to override these details for IBM media, you can define media information to DFSMSrmm using the MEDINF command in the DFSMSrmm parmlib.

The following information is recorded or calculated:

Volume capacity: The Capacity (MB) field displays the physical capacity of the tape volume as reported by the hardware. The value is displayed in megabytes for those devices which provide capacity information, such as the IBM 3590 Magstar® and later devices. When not reported by the hardware, the capacity is derived from the media type and recording format. These values are hard-coded in DFSMSrmm or can be specified by MEDINF in parmlib.
Percent full/Percent of volume: The Percent full field displays how much of the volume has been taken up by the data written to it. The value is calculated using the reported position on the volume where the end of the last file was written. When not reported by the hardware DFSMSrmm uses the volume capacity and the physical bytes written to the volume after compression to calculate the percent full. When data is written to a volume the bytes written are reported by the hardware in 4KB increments and so, when volume position is not reported, the displayed values are approximations of actual values. The Percent of volume field displays how much of the physical media space is occupied by the data set or file.
Compression: The compression field displays the compression ratio for the data your application wrote to the volume. It is calculated using ApplicationBytesWritten/DeviceBytesWritten and displayed to an accuracy of 2 decimal places. For example, a compression value of ‘3.33’ is a ratio of 3.33:1. Consider that ApplicationBytesWritten is the data set size field, and for RECFM VB is an approximation. Also DeviceBytesWritten, displayed as the data set physical size, may be rounded to 1KB or 4KB increments depending on the hardware involved. As a result, the reported compression value can fluctuate significantly with smaller data set sizes. If no compression ratio can be computed, a value of 0.00. is reported.
Data set size: This is an estimation of how much data was written by the application prior to any compression/compaction. The Block Count field specifies the number of blocks written to the tape volume. The bock count value displayed corresponds to that recorded in the data set's End-of-File label. The Block Size and the Block Count are used to estimate the Size of the data set.
Volume usage: The Volume Usage field displays how much of the volume has been taken up by the data sets that reside on it. The volume usage is the sum of the size of all data sets on the volume.
Physical size: The data set physical size displays how much data is reported to have been written by the tape drive to the volume and is reported after any compression. The values reported depend on the tape hardware used. IBM tape drives defined as 3590-1 report using 1KB increments. Older IBM tape drives such as 3490 and virtual tape emulating 3490 report in 4KB increments.
Physical used: The volume physical used field displays the sum of the physical size of all files on the volume. Many small files on a 3490 can potentially distort the reported values. For IBM tape drives that report using 1KB increments this value accurately reflects the physical tape usage and is not a sum of all the tape files physical size.

Accurate recording of information depends on all files on a volume being created and updated by a z/OS® release that supports recording of physical size:

When data was written prior to the support for tracking physical size/usage DFSMSrmm displays no value (0KB).
When data was written on a supporting level, a valid physical size is displayed.
In case of a volume containing a mix of data sets created before the new function and some created with the new function, each individual data set record displays values as listed above, however the displayed volume physical used and percentage full values will be incorrect because they can only reflect the sum of the data set values that are actually available.

LISTVOLUME and LISTDATASET display the values recorded by DFSMSrmm. Refer to Extract file processingfor how to get better information.