Explanation: DUMPIO(n,m) is an optional
parameter identifying which DFSMSdss DASD I/O buffering technique
to use and the number of tracks to read for each EXCP.
The n indicates the DFSMSdss DASD I/O buffering technique
for reading tracks. This value applies to DFSMSdss physical volume
dump, when using the BACKVOL command with the DUMP subparameter.
The m indicates the value used for DFSMSdss logical dump.
If m is not specified, the value will be the same as n.
The values to be used for
n and
m and their meanings
are:
- Value
- Meaning
- 1
- DFSMSdss reads 1 track at a time
- 2
- DFSMSdss reads 2 tracks at a time
- 3
- DFSMSdss reads 5 tracks at a time
- 4
- DFSMSdss reads 1 cylinder at a time
Note: - Exercise caution when using the DUMPIO parameter because it can
have a severe effect on virtual and real storage requirements in a z/OS® system. The higher the level
of optimization used, the more real storage required for I/O buffers,
which are page-fixed. This could result in a limit to the number
of concurrent DFSMShsm tasks, could require that DFSMShsm address
space be expanded, or could cause overall system degradation. Refer
to z/OS DFSMShsm Implementation and Customization Guide for
information on how to calculate storage requirements.
- DFSMSdss uses three I/O buffers for each active dump task. The
size of each of these buffers is determined by the DASD device type
and by how many tracks or cylinders are specified in the DUMPIO parameter.
For example, if you specify SETSYS DUMPIO(4) and SETSYS MAXDUMPTASKS(15),
DFSMSdss gets buffers for 3 cylinders of data for each dump task that
is running. The total storage required, if you are using a 3380 DASD
device, is 33MB of 31-bit storage. This means that the operating system
region size must be larger than the default of 32MB, or an ABEND878
occurs.
- One other performance consideration is that each page
of buffer storage will be page-fixed, requiring time and machine cycles.
- DUMPIO does not apply during aggregate backup and recovery, use
the SETSYS ABARSOPTIMIZE parameter instead.
SMS relationship: Parameter has the same
meaning when applied to SMS-managed or non-SMS-managed DASD volumes
or data sets.
SETSYS default: If you specify DUMPIO without
a value for m, then m defaults to n.
DFSMShsm default: If you do not specify this
parameter on any SETSYS command, the DFSMShsm default for n is
one and the default for m is four.