Overflow storage groups

Overflow storage groups are a new type of pool storage group introduced with z/OS V1R3 DFSMS. They are used for the initial data set creation when there is insufficient space available in the primary storage group for the first extent of a data set. If there is an overflow storage group defined and the ACS routines allow the use of the overflow storage group for this data set, a data set will be directed to the overflow storage group if all volumes in the primary storage groups are over their high allocation threshold.

Many customers currently achieve a similar result by using storage groups or volumes defined to SMS in QUINEW status. These are often called spill storage groups. The difference between spill and overflow storage groups is that with spill storage groups, the volumes need to be in quiesced status, and volumes in an overflow group can be defined as enabled.

Figure 2-12 shows a section of the JOBLOG output of a data set allocation that was directed directly to an overflow storage group.

Figure 2-12 Allocation directed to an overflow storage group

Overflow storage groups are only considered for the initial creation of a data set, they are not seen by EOV processing.

There are two steps to consider when implementing an overflow storage group: 򐂰 Defining the overflow storage group to ISMF

򐂰 Adding the storage group to your storage group ACS routine

Note: The storage group that contains the volume that was successfully allocated to becomes the primary storage group. This means that the overflow storage group can become the data set’s primary storage group.

IGD17223I JOBNAME (JMEREP01) PROGRAM NAME (IKJEFT01) STEPNAME (S1 ) DDNAME (OUT ) DATA SET (MHLRES4.TESTC.OVRF1 ) WAS ALLOCATED TO AN OVERFLOW STORAGE GROUP SGMHL02

2.4.1 Defining the overflow storage group

You define a storage group as an overflow storage group using the ISMF Storage Group Application to either define a new storage group or alter an existing one. We show the POOL STORAGE GROUP ALTER panel in Figure 2-13. All that is required at the storage group is to specify Overflow . . . Y, the default value for this field is Overflow . . . .N. Any pool storage groups defined prior to z/OS V1R3 DFSMS are designated as non-overflow by default.

Figure 2-13 Defining an overflow storage group

If you have a mixture of operating system levels in your SMSplex and wish to define overflow storage groups on those systems at z/OS V1R3 DFSMS, we recommend that the status of this group be set to QUINEW on all down-level systems as they will not recognize the overflow status. There are no toleration PTFs available.

We recommend that you use a one-to-many relationship between your overflow and normal pool storage groups. An overflow storage group must be a pool storage group. You can use it for non-overflow allocations but each data set directed to the overflow storage group will receive message IGD17223I in both JOBLOG and SYSLOG. Overflow storage groups are only considered during the initial creation of a data set. Once the data set begins to extend only the primary and extend storage groups, if present, are available.

Panel Utilities Help ssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss DGTDCSG2 POOL STORAGE GROUP ALTER Command ===> SCDS Name . . . : SYS1.SMS.SCDS Storage Group Name : SGMHL02 To ALTER Storage Group, Specify: Description ==> TESTING FOR SMS 1.3 RESIDENCY ==> Auto Migrate . . Y (Y, N, I or P) Migrate Sys/Sys Group Name . . Auto Backup . . Y (Y or N) Backup Sys/Sys Group Name . . Auto Dump . . . N (Y or N) Dump Sys/Sys Group Name . . . Overflow . . . . Y (Y or N) Extend SG Name . . . Dump Class . . . (1 to 8 characters) Dump Class . . . Dump Class . . . Dump Class . . . Dump Class . . . Allocation/migration Threshold: High . . 85 (1-99) Low . . 5 (0-99) Guaranteed Backup Frequency . . . NOLIMIT (1 to 9999 or NOLIMIT) ALTER SMS Storage Group Status . . . N (Y or N) Use ENTER to Perform Verification and Selection;

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Once a storage group has been defined as an overflow storage group, you must update your storage group ACS routines to add the overflow storage group. In Figure 2-14 we show a fragment of the ACS routines that we used to allow allocations to flow to an overflow storage group. Our overflow storage group was SGMHL02.

Figure 2-14 Storage group ACS routine assigning an overflow storage group

Storage groups defined with Overflow . . . . Y can also be used as extend storage groups for other pools.

When an overflow pool storage group contains more volumes than a non-overflow pool storage group, specified volume counts might result in volumes in the overflow storage group being preferred over volumes in the pool storage group during volume selection. This is just probability as the primary pools near their high allocation thresholds. We discuss this in more detail in “Summary of factors influencing volume selection” on page 35.

2.4.2 Using overflow storage groups

If you use storage groups in QUINEW status today, these should be converted to overflow storage groups. Volumes in an overflow storage group are preferred to volumes with either a volume status or storage group status of QUINEW.

PROC 0 STORGRP SELECT(&DSN) WHEN(MHLRES4.TESTB.**) DO SET &STORGRP EQ 'SGMHL02' EXIT CODE(0) END WHEN(MHLRES4.TESTC.**) DO SET &STORGRP EQ 'SGMHL03','SGMHL02' EXIT CODE(0) END WHEN(MHLRES4.TESTD.**) DO SET &STORGRP EQ 'SGMHL04' EXIT CODE(0) END OTHERWISE DO SET &STORGRP = 'SGMHL01' EXIT CODE(0) END END /* END SELECT */

Volumes from the overflow storage group will only be selected by allocation if there are insufficient volumes under their high allocation threshold value in any of the eligible non-overflow storage groups. Once an extent has been allocated on a volume in a non-overflow storage group, volumes in an overflow storage group are not considered by extend processing as targets unless the overflow storage group is also defined as an extend storage group.

Similarly, if the initial allocation is made to a volume in an overflow storage group, then all subsequent data set extents will be taken either in the overflow storage group or its extend storage group if one has been defined. The volumes in the ACS eligible storage groups will not be considered for extend processing since the overflow pool storage group becomes the primary storage group in this case. Unlike extend storage groups, overflow storage groups will contain the initial extent of a data set. You need to ensure that any space management or

availability management functions enabled on the primary pool are also enabled on the overflow storage group.

If you are relying on full volume processing for data archiving or recovery, you must ensure that data sets allocated to an overflow storage group are included. If you are using DFSMShsm to manage full volume dumps, you could either ensure that the overflow pool storage group is assigned the relevant dump classes, or you could use interval migration or command migration specifying CONVERT to move data back to the storage groups that should have initially contained this data.

Guaranteed space and overflow storage groups

If a data set is being allocated with a storage class specifing that guaranteed space and specific volume serials are supplied, the following rules apply: 򐂰 If the volume specified is in the overflow storage group, the allocation will be

honored as long as sufficient space exists on the selected volume to hold the primary extent of the data set, as the overflow pool storage group is one of those allowed by the storage group ACS routine.

򐂰 If the volume serial specified is not in the overflow pool storage group and there is not sufficient space on the selected volume to hold the primary extent of the data set, even after space constraint relief processing, then the allocation will fail.

If the storage class specifies guaranteed space and there is no explicit volume serial passed in the allocation, then the volumes in an overflow pool storage group are eligible to be considered for the data set allocation.

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