Disk space numbers about 3.5% off

Products

• SYSVIEW Performance Management

Issue/Introduction

We noticed that SYSVIEW space number in bytes/Mb/Gb are always a little off of the real numbers.

IBM documentation - Direct access storage devices states that on 3390 devices, there are 849,960 bytes per cylinder, but SYSVIEW calculates 881,790.

For example when allocating a file in MB units the following file:

SYSVIEW displays: 

This figure equals 27547Mb.

 

We noticed this when comparing the numbers with Vantage, which seems to have the right numbers.

 

 

Resolution

In the SYSVIEW 'SPACE' command, under options, you will see one set to 'NOTRKCALC' by default.  

If you change this to 'TRKCALC', the values will correspond with the values you are seeing in the Vantage product.  

 

This is because of how usable space is laid out on individual tracks.  

One track has the ability to store 58,786 Bytes of data.  This is the number of Bytes that SYSVIEW uses for calculation by default.  

Some of this space (a little over 2kB) on every track contains some IBM data used for indexing, addressing, etc.  

Vantage, among other products, display the total space on the track minus this IBM data, to show only usable space (documented as 56,664 Bytes).  

Setting the option NOTRKCALC to TRKCALC will tell SYSVIEW that you would like to see values based on (Total - IBM data) rather than just (Total).