2. COST ACCOUNTING CONCEPTS FOR IS ORGANIZATIONS › 2.7 DASD/DFHSM/SMS Storage Accounting › 2.7.4 DFHSM Backup Data Sets

2.7.4 DFHSM Backup Data Sets

DFHSM can automatically create backup copies of data sets
according to installation-defined rules.  System Managed
Storage (SMS) management class definitions can also be used
to specify backup frequency and the number of backup versions
that are to be kept.  When DFHSM performs automatic backup of
data sets, the Backup Control Data Set (BCDS) is updated with
information about each backup copy or backup version created
on backup volumes managed by DFHSM.

The pyramid forms a useful analogy for considering the
potential amount of data for backup versions.  Consider this
pattern of usage for typical TSO users and the backup rules
associated with a "STANDARD" management class:

  o 25 percent of a given user's data sets are changed each
    day.

  o An HSM rule states data sets are only to be backed up if
    they have been changed.

  o A management class definition states that five backup
    versions are to be kept.

Thus, for an active data set that is changed every day, the
BCDS will contain information for five backup versions of
this data set in our hypothetical example above.

Expanding this scenario to an entire DASD complex, there
might be the following kind of relationship in terms of the
relative number of data sets:

                             /\
                            /  \
                           /    \
                          /      \
                         /        \
                        /  Primary \
                       /____________\
                      /              \
                     /                \
                    /                  \
                   /       Backup       \
                  /       Versions       \
                 /                        \
                /                          \
               /____________________________\



Information on backup versions of data sets is available from
the CA MICS Space Analyzer Option HSM product via the CA MICS
Space Collector Option (VCC).  VCC can optionally collect
basic accounting and capacity planning information about
backup versions of data sets by serving as a "driver" for the
collection of data from the BCDS.

Using the CA MICS Space Analyzer Option HSM product, you can
establish different pricing for storage occupied for backup
versions.  Because the backup versions typically exist on
high-density tape cartridges, the relative charge for storage
occupancy (that is, amount of space times duration or
megabyte days) is undoubtedly less for backup versions than
for primary DASD storage occupancy.  An example follows:



                             /\
                            /  \
                           /    \
                          /      \
                         /        \
                        /          \
                       /            \
                      /              \
                     /     $1.25      \
                    /       per        \
                   /    megabyte day    \
                  /______________________\
                 /                        \
                /          $0.05           \
               /            per             \
              /         megabyte day         \
             /________________________________\



See Section 2.7.7, DASD Accounting Features, for more
information on this subject.