2. COST ACCOUNTING CONCEPTS FOR IS ORGANIZATIONS › 2.7 DASD/DFHSM/SMS Storage Accounting

2.7 DASD/DFHSM/SMS Storage Accounting

Since data storage is one of the most expensive areas of IS
organizations, chargeback to the user is crucial to the
economics and planning of the organization.  It is therefore
necessary to have an accurate measurement of utilization so
that users can be charged in a manner that encourages
efficient space management.

The two basic methods of direct access space accounting are
Volume Table of Contents (VTOC) snapshots and active
monitoring.  These methods differ in the accuracy of the data
captured and the overhead necessary to capture it.


VTOC SNAPSHOTS

By far the most common method of collecting data for direct
access space accounting is to read the VTOCs of the online
packs at regular intervals, usually once a day, and to assume
that the space utilization is relatively constant between
"snapshots." There are two problems with this method:

o It does not account for temporary space utilization, which
  may be a significant load center for the installation; and,

o Because the snapshot is generally taken during the off
  hours to minimize performance impacts, it accounts for only
  the minimum space utilization level.

However, the VTOC snapshot approach has several things to
recommend it:

o It is relatively inexpensive.

o It does not require coding of operating system
  modifications or exits.

o It does not incur the overhead of data capture for each
  Direct Access Device Space Monitor (DADSM) operation.

o It is easier to maintain than active monitoring.

This is the approach supported by the CA MICS Space Collector
Option and CA MICS Space Analyzer Option.

ACTIVE MONITORING

An alternative to snapshot processing is the active
monitoring approach.  With this methodology, a software
monitor determines space utilization by intercepting data set
maintenance functions as they are performed.  The keynotes of
this approach are accuracy and the ability to account for
temporary space utilization.

The problem with this approach is that the overhead of
collecting the data may not be recovered by the additional
accuracy that it allows, especially since the majority of the
overhead occurs during periods of heavy system use.  For this
reason, it is not as popular as the VTOC snapshot method.

The following sections discuss the different types of
DASD/DFHSM/SMS accounting:

  1 - Permanent DASD Space Accounting
  2 - Temporary DASD Space Accounting
  3 - DFHSM Migrated Data Sets
  4 - DFHSM Backup Data Sets
  5 - VSAM Accounting
  6 - Systems Managed Storage (SMS) Accounting
  7 - DASD Accounting Features