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Status Future consideration
Workspace z/OS
Categories DFSMS VSAM
Created by Guest
Created on Jun 16, 2023

RELATED IDEAS

Change Default For VSAM Files Definition Extent Constraint Removal To Yes

We experienced a major outage in a number of our critical production systems late last year due to a Db2 partition hitting max extents before it could hit the maximum defined data set size.  Db2's space management algorithm was to scan the full dataset over and over again looking for free space rather than just define a new dataset.  We were told by Db2 suport that it would be far less likely for us to have encountered this problem if we had Extent Constraint Removal set to Yes in the data class for the dataset in question.  We have since started a project to convert most of our Db2 files to use Extent Constraint Removal to Yes but with over 2 million tablespace files it will take us years as it not only includes a change to a new SMS data class but for a Db2 reorg to be executed to delete and redefine the file.  We discussed this with Db2 development and were asked to open this idea for DFSMS to make this a default setting with the end result being that every existing VSAM file that was not explicitly defined with Extent Constraint Removal as No would then be eligible to extend to the theoretical limit of 7257 extents.  The Db2 for z/OS Aha! Idea for the issue with Db2 is DB24ZOS-I-1414 if you want more details from the Db2 side of the house.

Idea priority Urgent
  • Guest
    Reply
    |     Dec 7, 2023
    Reviewing the possibility of a change in development release.
  • Guest
    Reply
    |     Sep 6, 2023

    The data class documentation in DFSMSdfp Storage Administration says "If you alter a data class definition, SMS applies the changes to any new data sets that use the data class after you activate the changed configuration. However, SMS does not retroactively apply your changes to previously allocated data sets. To apply your changes to existing data sets, you need to delete and redefine the data sets."

    According to the previous comment, that is not true for the three extent-constraint removal options and the book should be corrected to cite this exception.

  • Guest
    Reply
    |     Sep 5, 2023
    If I am understanding you correctly, you have created a new data class with Extent Constraint Relief (ECR) set to YES and you are planning on reorging your Db2 data tables to specify the new data class. ECR is not one of the attributes that is stored with the data set, instead it is read at extent time. So, another way to accomplish what I think you want would be to simply modify your existing data classes to have ECR set to YES and the new behavior will begin on the next extent for any data set using that data class. That may not help you on the one that is already out of extents, but that should simplify your project greatly.
    I don't know that everyone would agree that the default should be YES, since then you would have the opposite problem of non-critical applications potentially being able to extend to the point of impacting critical work loads. I've been watching the votes and so far no others have voted for this. So at this point I will decline this, but if you want to try for more votes you can submit to the ISMF group since VSAM does not control or store this value.
  • Guest
    Reply
    |     Jul 13, 2023

    I would expect this change to the DATACLAS definition itself should take effect without the need for a reorg of the dataset because it is not changing any formatting or compression attributes. The dataset may possibly need to closed and reopened to refresh the in-storage control blocks to reflect the change. You certainly have to redefine to pick up a new DATACLAS as the existing one assigned cannot be changed, but this is not changing the assigned DATACLAS. I am sure we have changed the DYNVOL values in DATACLASes before without any issues but I can't recall if it affected DB2 datasets.

  • Guest
    Reply
    |     Jun 22, 2023

    The system applies most options in data class only when a data set is created. Subsequent changes to the data class definition have no effect on existing data sets. This proposed option would be an exception.

    Also keep in mind that each volume can hold no more than 123 extents. In order to have 7257 extents, the data set must reside on 59 volumes and every volume was able to get 123 extents.

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