Selecting files for use as data tables

It is not possible to lay down any exact rules about whether a file will benefit from conversion to a shared data table. There are many considerations, and an analysis of the potential uses of shared data tables support should be undertaken by someone who understands how the files are used by the various applications and the configuration of the CICS® regions.

The following checklist gives some general guidance. Additional sources of information that could help you to select the files include:

However, the most beneficial input to the selection process is a thorough understanding of the applications and the way in which they use the files.

Checklist

If your installation is using data tables for the first time, the following checklist gives some general principles to help you select files for defining as data tables.

Using statistics to select data tables

This section covers just one of the possible inputs to the selection task--the information available from the file statistics.

If you need to share data between more that one MVS™ image, you should investigate using RLS mode to share the files. If, however, your sharing is confined to a single MVS image, you should consider which files have access patterns that make the use of shared data tables beneficial.

Figure 3, Figure 4, and Figure 5 show some extracts from a hypothetical set of file statistics for files accessed in non-RLS mode that are used in the following discussion to demonstrate how CICS statistics can aid the selection process.

The statistics are displayed as they would be reported by the CICS offline formatting utility. Requested file statistics are shown, but Interval or End of Day statistics would be equally suitable. The section of File "Performance Information" statistics, which reports use of VSAM strings and buffers, is not shown here.

The numbers shown in the figures are purely for the purposes of illustration, and you should not expect the statistics at your installation to resemble them. Similarly, the configuration of CICS regions and files has been chosen to highlight certain points; it is not suggested that this is a typical or desirable configuration.

Interpreting data table statistics discusses the statistics reported for files defined as data tables, which you can use to assess the benefits being obtained.

Figure 3. CICFOR requested file statistics
Requested Statistics Report        Collection Date-Time 12/25/99-11:51:51  Last Reset 09:00:00  Applid CICFOR    Jobname SDTGSTF1
_________________________________________________________________________________________________________________________________
FILES - Resource Information
____________________________
  File      Dataset Name                               Dataset   RLS      DT      Time     Time     Remote    Remote   Lsrpool
  Name          Base Dataset Name (If Applicable)       Type     File  Indicator Opened   Closed     Name     Sysid      ID
  _____________________________________________________________________________________________________________________________
  APPLE     CIC01.CICOWN.APPLES                          K       NO              07:44:12  OPEN                           1
  BANANA    CIC01.CICOWN.BANANAS                         K       NO              09:45:08  OPEN                           1
  ORANGE    CIC01.CICOWN.CITRUS                          K       NO              10:51:10  OPEN                           2
  PEAR      CIC01.CICOWN.PEARS                           K       NO              07:30:14  OPEN                           3
  _____________________________________________________________________________________________________________________________
Requested Statistics Report        Collection Date-Time 12/25/99-11:51:51  Last Reset 09:00:00  Applid CICFOR    Jobname SDTGSTF1
_________________________________________________________________________________________________________________________________
FILES - Requests Information
____________________________
  File     Get      Get Upd  Browse   Update   Add      Delete   Brws Upd VSAM EXCP Requests RLS req
  Name     Requests Requests Requests Requests Requests Requests Requests   Data    Index    Timeouts
  ____________________________________________________________________________________________________
  APPLE     2317265     1020        0     1019       21        1        0     11503      310        0
  BANANA     536452     1674    20344     1674      908        0        0      2651       70        0
  ORANGE    2069454    98560    17831    98327     4543     2563        0      8511      481        0
  PEAR        45871    65493     6512    65493    30109      362        0      3773      231        0
  ____________________________________________________________________________________________________
  *TOTALS*  4969042   166747    44687   166513    35581     2926        0                           0
Requested Statistics Report        Collection Date-Time 12/25/99-11:51:51  Last Reset 09:00:00  Applid CICFOR    Jobname SDTGSTF1
_________________________________________________________________________________________________________________________________
FILES - Data Table Requests Information
_______________________________________
  File     Close    Read       Recs ¬   Adds from Add      Adds rejected Adds rejected Rewrite  Delete   Highest    Storage
  Name     Type     Requests   in Table   Reads   Requests    - Exit     - Table Full  Requests Requests Table Size Alloc(K)
  ___________________________________________________________________________________________________________________________
DFHST0223 I There are no data table statistics to report.

Figure 4. CICAOR1 requested file statistics
Requested Statistics Report        Collection Date-Time 12/25/99-11:51:38  Last Reset 09:00:00  Applid CICAOR1   Jobname SDTGSTA1
_________________________________________________________________________________________________________________________________
FILES - Resource Information
____________________________
  File      Dataset Name                               Dataset   RLS      DT      Time     Time     Remote    Remote   Lsrpool
  Name          Base Dataset Name (If Applicable)       Type     File  Indicator Opened   Closed     Name     Sysid      ID
  _______________________________________________________________________________________________________________________________
  APPLE     REMOTE                                                               CLOSED   CLOSED   APPLE     CIF1        N
  BANANA    REMOTE                                                               CLOSED   CLOSED   BANANA    CIF1        N
  ORANGE    REMOTE                                                               CLOSED   CLOSED   ORANGE    CIF1        N
  ZUCCHINI  REMOTE                                                               CLOSED   CLOSED   COURGETT  CIA2        N
  _______________________________________________________________________________________________________________________________
Requested Statistics Report        Collection Date-Time 12/25/99-11:51:38  Last Reset 09:00:00  Applid CICAOR1   Jobname SDTGSTA1
_________________________________________________________________________________________________________________________________
FILES - Requests Information
____________________________
  File     Get      Get Upd  Browse   Update   Add      Delete   Brws Upd VSAM EXCP Requests RLS req
  Name     Requests Requests Requests Requests Requests Requests Requests   Data    Index    Timeouts
  ____________________________________________________________________________________________________
  APPLE     1158701      532        0      531       11        1        0         0        0        0
  BANANA     305641        0    19067        0        0        0        0         0        0        0
  ORANGE      58709    32854     4265    32621     1018     1001        0         0        0        0
  ZUCCHINI    78914        0    14765        0        0        0        0         0        0        0
  ____________________________________________________________________________________________________
  *TOTALS*  1601965    33386    38097    33152     1029     1002        0                           0
Requested Statistics Report        Collection Date-Time 12/25/99-11:51:38  Last Reset 09:00:00  Applid CICAOR1   Jobname SDTGSTA1
_________________________________________________________________________________________________________________________________
FILES - Data Table Requests Information
_______________________________________
  File     Close    Read       Recs ¬   Adds from Add      Adds rejected Adds rejected Rewrite  Delete   Highest    Storage
  Name     Type     Requests   in Table   Reads   Requests    - Exit     - Table Full  Requests Requests Table Size Alloc(K)
  ___________________________________________________________________________________________________________________________
DFHST0223 I There are no data table statistics to report.

Figure 5. CICAOR2 requested file statistics
Requested Statistics Report        Collection Date-Time 12/25/99-11:49:31  Last Reset 09:00:00  Applid CICAOR2   Jobname SDTGSTA2
_________________________________________________________________________________________________________________________________
FILES - Resource Information
____________________________
  File      Dataset Name                               Dataset   RLS      DT      Time     Time     Remote    Remote   Lsrpool
  Name          Base Dataset Name (If Applicable)       Type     File  Indicator Opened   Closed     Name     Sysid      ID
  _______________________________________________________________________________________________________________________________
  COURGETT  CIC02.CICOWN.COURGETT                        K       NO              08:22:15  OPEN                           1
  LEMON     REMOTE                                               NO               CLOSED   CLOSED   ORANGE    CIF1        N
  _______________________________________________________________________________________________________________________________
Requested Statistics Report        Collection Date-Time 12/25/99-11:49:31  Last Reset 09:00:00  Applid CICAOR2   Jobname SDTGSTA2
_________________________________________________________________________________________________________________________________
FILES - Requests Information
____________________________
  File     Get      Get Upd  Browse   Update   Add      Delete   Brws Upd VSAM EXCP Requests RLS req
  Name     Requests Requests Requests Requests Requests Requests Requests   Data    Index    Timeouts
  ____________________________________________________________________________________________________
  COURGETT    78914    27469    14765    27469   336472        0        0      8212      481        0
  LEMON     2010745    65706    13566    65706     3525     1562        0         0        0        0
  ____________________________________________________________________________________________________
  *TOTALS*  2089659    93175    28331    93175   339997     1562        0                           0
Requested Statistics Report        Collection Date-Time 12/25/99-11:49:31  Last Reset 09:00:00  Applid CICAOR2   Jobname SDTGSTA2
_________________________________________________________________________________________________________________________________
FILES - Data Table Requests Information
_______________________________________
  File     Close    Read       Recs ¬   Adds from Add      Adds rejected Adds rejected Rewrite  Delete   Highest    Storage
  Name     Type     Requests   in Table   Reads   Requests    - Exit     - Table Full  Requests Requests Table Size Alloc(K)
  ___________________________________________________________________________________________________________________________
DFHST0223 I There are no data table statistics to report.

The examples use a hypothetical configuration of three CICS regions. Most of the files used by CICS applications are owned by the file-owning region CICFOR, and the applications mostly run in the application-owning regions CICAOR1 and CICAOR2. This discussion assumes that each of the data sets shown in the statistics reports is a VSAM base KSDS (as indicated by the Dataset Type of K), so any of them can be defined as data tables.

This section focuses on identifying candidates for defining as CICS-maintained data tables, because the decision to define a user-maintained data table is more likely to come from consideration of particular applications than from a study of file performance in general. Because of this focus, none of the statistics shown is for files accessed in RLS mode, because an RLS-mode data set cannot be the source for a CICS-maintained data table.

The statistics also show you which file names in one region are defined to access file names in another region. The Remote Sysid is the name given on the connection between the two regions. In the examples, the SYSID of CICFOR is CIF2 and that of CICAOR2 is CIA2.

A file with a high read-to-update ratio

The file APPLE is used by applications that run on the application-owning region CICAOR1. It is defined in CICAOR1 as a remote file, and the file definition points to the file APPLE owned by CICFOR. This file would benefit from being redefined in CICFOR as a CICS-maintained data table because it has a high ratio of remote reads (1158701 Get Requests in the time period covered by the reports) to remote updates (11 adds, 1 delete and 531 updates) as seen in Figure 4.

See the CICS Performance Guide for guidance on the meanings of the "FILES - Requests Information" section of a statistics report.

A file with a high proportion of remote reads

The file BANANA is updated and read on CICFOR, but is also accessed by CICAOR1. Because all the remote accesses are reads and browses, with no updates, the applications running in CICAOR1 would probably see large benefits if BANANA was defined as a data table, and the applications on CICFOR would also benefit by reading from the local data table.

A file shared by several regions

From a study of the statistics in Figure 4 it might appear that ORANGE is not an especially suitable data table candidate, as the numbers of remote retrievals from CICAOR1 (58709 Get Requests and 4265 Browse Requests) are relatively low. However, the remote file LEMON in CICAOR2 also points to ORANGE in CICFOR, so defining ORANGE in CICFOR as a shared CICS-maintained data table would probably benefit the performance of the applications in both AORs.

A good UMT candidate

The file COURGETT owned by CICAOR2 is accessed via the filename ZUCCHINI in CICAOR1. CICAOR1 only reads or browses the file; any updating is issued by the owning region. Also, it is known that these updates are relevant only to the day’s CICS run and do not need to be retained permanently (in fact, they are deleted at shutdown). The file is therefore an excellent candidate for defining as a user-maintained data table. All the updates can then be made to the data table without any VSAM I/O activity, and all the remote retrievals can be made without function shipping.

A rather poor candidate

The file PEAR would probably not benefit much from shared data tables support because it is not accessed remotely and has many update and browse requests. Local browsing does not offer as much benefit as either local reading or any form of remote retrieval, because VSAM browsing (apart from processing of the STARTBR command) is very efficient. This analysis, of course, does not consider the relative importance of the various file accesses; the reading might be done by critical applications, but the time taken for updates might not be important.

Other possible candidates

The preceding examples illustrate only a small sample of the possible configurations and uses of files that could benefit from shared data tables support.

You could also use shared data tables support to avoid the need to duplicate files or data tables in each region. And, in addition to looking at existing files, you could consider moving files from an AOR to an FOR where this was not practical before because of the cost of file accesses using function shipping.

Related concepts
File control
File statistics
Planning to use data tables
Performance benefits of data tables
Security checking for data tables
Shared data tables support on different releases of CICS
Preparing to use shared data tables support
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