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Health Check CA_DTCM,DTCM_DB_CF_LIST_STR_SZ Coupling Facility structure size recommendation

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Article ID: 257264

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Updated On:

Products

Datacom/AD Datacom Datacom/DB

Issue/Introduction

Why would Health Check CA_DTCM,DTCM_DB_CF_LIST_STR_SZ be recommending such a large Coupling Facility (CF) structure size:

     Based on usage and current configuration CA recommends defining list
     structure CADTCMADXPLEX_L1 with a minimal size of 73728 (in 1k
     blocks). This is done by altering the CRFM policy for this list
     structure for the Coupling Facility named ICF      , and specifying
     SIZE(73728) .

For our Datacom/AD Shadow environment it seems excessive.
Currently the  structure is defined with a size of 16K and a  MIGRATE_TO_SHADOW command executed successfully. 

Full Health Check text:

 SYSPLEX:    PRODPLEX  SYSTEM: SYS1
 START TIME: 12/12/2022 12:20:19.418403
 CHECK DATE: 20151231  CHECK SEVERITY: HIGH-DYNAMIC
 CHECK PARM: THRESHOLD(90)


 CF List Structure        Size In    Number of  Number of
 CADTCMADXPLEX_L1         1k Blocks  Entries    Elements

 -----------------------   --------   --------   --------
 Capacity if Defined          16384       1475      20344
 Max Usage if Connected         N/A         77        193
 By Configuration 2 MUFs      65536       6635     190682

 * High Severity Exception *

 DBH6005E Risk of MUFPLEX failure because Coupling Facility structure
 CADTCMADXPLEX_L1  is too small.

   Explanation:  You specified MUFPLEX mode S in the Multi-User startup
     options. This allows this physical instance of the Multi-User
     Facility (MUF) to have a Shadow which can be used to minimize any
     loss of data access if a failure of this physical instance of MUF,
     or some other outage occurs.

     During planned outages in shadow mode, you can temporarily use a
     full MUFPLEX to migrate from one physical instance of MUF to
     another. This full MUFPLEX environment uses a coupling facility list
     structure. The size of this structure, as well as the number of
     entries and elements must be sufficient to handle the amount of
     concurrently shared data, as well shared Datacom control blocks.

     You are not currently in this full MUFPLEX mode.

     In list structure CADTCMADXPLEX_L1 the following were considered
     unhealthy based on actual usage or potential usage based on the
     MUFPLEX configuration:
     structure size, number of entries, number of elements.

     A configuration is based on the high water usage for this physical
     instance of MUF times two (two MUFs in the MUFPLEX). It is possible
     that you may never need to share the entire configuration.

   System Action:  Processing continues until the structure is truly full
     at which point errors will occur which can include the failure of
     the entire MUFPLEX.

   Operator Response:  Notify the DBA of this condition.

   System Programmer Response:  The MUFPLEX Multi-User startup option has
     a value of S. This indicates you are running in Shadow mode.
     Typically the only time you need the the coupling facility structure
     in this mode is when you are migrating from one Multi-User Facility
     to another due to a "planned outage". This is done by issuing an
     ALTER_SHADOW_TO_MUFPLEX or MIGRATE_TO_SHADOW command. You are not
     currently in this altered mode.

     Based on usage and current configuration CA recommends defining list
     structure CADTCMADXPLEX_L1 with a minimal size of 73728 (in 1k
     blocks). This is done by altering the CRFM policy for this list
     structure for the Coupling Facility named ICF      , and specifying
     SIZE(73728) .

     The list structure's entry and element ratios are specified on the
     MUFPLEX Multi-User startup option. CA recommends specifying
     MUFPLEX_ENTRY_RATIO as 1 and MUFPLEX_ELEMENT_RATIO as 2 based on
     your usage and current configuration. This startup option can be
     found in the Multi-User Sysin. The entry ratio is the 7th parameter,
     and the element ratio is the 8th parameter on the MUFPLEX startup
     option. Incorrect ratios can result in a list structure filling in
     spite of the structure size.

   Problem Determination:  Refer to the CA Datacom/DB Database and System
     Administration Guide.

   Source:  CA Datacom/DB, DBH02PR

   Reference Documentation:  CA Datacom/DB Database and System
     Administration Guide following sections: Modifying MUF Startup
     Options"  MUFPLEX, "Parallel Sysplex, Datasharing MUFplex", and
     "Shadow MUF Environment".

   Automation:  N/A

   Check Reason:  Ensures Coupling Facility list structure size is
     sufficient.

 END TIME: 12/12/2022 12:20:19.419857  STATUS: EXCEPTION-HIGH

Environment

Release : 15.1

Resolution

There is no minimum recommended size for the Coupling Facility structures. The size of the structure depends on the MUF configuration, the number of TASKS, EXCTLNO and buffers.

The  MIGRATE_TO_SHADOW command executed successfully with a 16k structure because there was little activity at the time.

The TASKS MUF startup option is defined with 250 tasks so potentially there could be a lot of activity at any one time.

The method of calculating the structure sizes is documented in section Coupling Facility Strategies for MUFplex under heading Calculating Structure Sizes.

However, the Health Check does this at runtime it calculates the required structure sizes based on the MUF configuration and the actual usage. So whatever it recommends is the most appropriate Coupling Facility structure size for that MUF.

 

Additional Information

If using a shadow MUF not a full MUFplex, a Primary/Shadow MUF can run without a Coupling Facility structure. The only time a Shadow MUF needs a coupling facility structure is to perform a planned outage, which requires the use of ALTER_SHADOW_TO_MUFPLEX or MIGRATE_TO_SHADOW commands. In this case both the Primary and Shadow MUFs temporarily become full MUFplex MUFs at the same time and require use of the coupling facility locking structures.

If a planned outage is not required, specify a mode of S_NO_STRUCTURE in the MUFPLEX startup option to run the Shadow MUF more efficiently without a Coupling Facility structure.

MUFPLEX   ADFPLEX,*,,300,S_NO_STRUCTURE