Because DFSMShsm can use both tape and DASD media types for data
set backup requests, there is a need to balance existing workloads
against available or allowed resources. In general, tapes take longer
from initial selection to first write than DASD does, but tape may
be faster in terms of data throughput. By tailoring the SETSYS DSBACKUP
command to define the number of tasks that are available to the data
set backup function, you can ensure that DFSMShsm efficiently directs
your backup to either tape or DASD, depending on the workload.
The DASDSELECTIONSIZE (maximum standard) parameter
on the SETSYS DSBACKUP command allows DFSMShsm to balance the workload
between DASD and tape tasks for all WAIT requests that do not target
a device type. DFSMShsm uses a selection process that is based on
data set sizes and the availability of tasks. If the data set is greater
than a specified threshold maximum, then the
data set is directed to tape. Any data set that is less than or equal
to a specified threshold standard is directed
to the first available task, either DASD or tape.
Note: If you specify the subparameter TASKS(0) for either
DASD or tape, DFSMShsm never directs nontargeted requests to that
media.
There are three distinct data set size ranges:
- A small data set is any data set with a
size less than the value of standard in DASDSELECTIONSIZE(maximum standard).
- A medium data set is any data set larger
than a small data set but less than the value of maximum in DASDSELECTIONSIZE(maximum standard).
- A large data set is any data set that is
greater than a medium data set.
Note: - There is a direct correlation between the number of defined tasks
(tape and DASD) and the size of data sets that are directed to DASD.
This correlation can result in double data movement during the automatic
backup function. (See Assigning DASDSELECTIONSIZE values for help with assigning
size values to maximum and standard.) The automatic backup function can delay the start of the multitasked
volume backup function; that is, volume backup will not begin until
all backup versions on ML1 have been moved to tape. You can use the
FREEVOL ML1BACKUPVERSIONS command to initiate movement of all backup
versions off all ML1 volumes to daily backup volumes.
- DFSMShsm starts tape tasks immediately on receipt of WAIT requests
(medium and large) and NOWAIT requests, up to the TAPE (DEMOUNTDELAY(MAXIDLETASKS))
value.
- When both tape and DASD tasks are allowed, NOWAIT requests are
only processed by tape tasks.
- DFSMShsm starts DASD tasks and tape tasks up to the allowable
value when there is enough work for the tasks to do. New tasks are
started that are based on the number of requests not currently being
processed.