When DFSMShsm is backing up or migrating data sets, it must prevent
those data sets from being changed. It does this by serialization.
Serialization is the process of controlling access to a resource to
protect the integrity of the resource. DFSMShsm serialization is determined
by the SETSYS DFHSMDATASETSERIALIZATION | USERDATASETSERIALIZATION
command.
Note: In DFSMS/MVS Version 1 Release 5, the incremental
backup function has been restructured in order to improve the performance
of that function. The SETSYS DFHSMDATASETSERIALIZATION command disables
that improvement. Only use the SETSYS DFHSMDATASETSERIALIZATION command
if your environment requires it. Otherwise, it is recommended that
you use the SETSYS USERDATASETSERIALIZATION command.
| If you specify . . . |
Then . . . |
|---|
| SETSYS DFHSMDATASETSERIALIZATION |
DFSMShsm issues a RESERVE command
that prevents other processing units from accessing the volume while DFSMShsm is
copying a data set during volume migration. To prevent system interlock, DFSMShsm releases
the reserve on the volume to update the control data sets and the
catalog. After the control data sets have been updated, DFSMShsm reads
the data set VTOC entry for the data set that was migrated to ensure
that no other processing unit has updated the data set while the control
data sets were being updated. If the data set has not been updated,
it is scratched. If the data set has been updated, DFSMShsm scratches
the migration copy of the data set and again updates the control data
sets and the catalog to reflect the current location of the data set.
Multivolume non-VSAM data sets are not supported by this serialization
option because of possible deadlock situations. For more information
about volume reserve serialization, see DFHSMDATASETSERIALIZATION. |
| SETSYS USERDATASETSERIALIZATION |
Serialization is maintained throughout
the complete migration operation, including the scratch of the copy
on the user volume. No other processing unit can update the data
set while DFSMShsm is performing its operations, and no second read of
the data set VTOC entry is required for checking. Also since there
is no volume reserved while copying the data set, other data sets
on the volume are accessible to users. Therefore, USERDATASETSERIALIZATION
provides a performance advantage to DFSMShsm and users in those systems
equipped to use it. |
You may use SETSYS USERDATASETSERIALIZATION if:
- The data sets being processed are only accessible to a single z/OS image,
even if you are running multiple DFSMShsm hosts in that single z/OS image.
OR
- The data sets can be accessed from multiple z/OS images, and
a product like GRS must be active and is required in a multiple-image
environment.
z/OS DFSMShsm Implementation and Customization Guide
Copyright IBM Corporation 1990, 2014