Overrides for IAM file processing
IAM has an override facility that provides easy access to many IAM special features. Some of the more common reasons for using IAM Overrides include:
- Adjusting the range of buffers for IAM Real Time Tuning
- Placing the buffers in 64-bit addressable storage.
- Enabling use of the above the bar cache for overflow blocks with the CACHE64 override
- Enabling use of the IAM Dynamic Data Space or DYNCORE Dynamic Tabling feature
- Enabling use of IAM’s Index Space
- Enable or disable the IAM journaling
- Altering values for IAM’s Dynamic Region Adjustment
- Activating the BACKUPCOMPRESSED feature
- Altering IAM’s Dynamic Secondary Space adjustment feature values
How to use overrides ?
Use of the IAM Override facility is easy. Add an IAMOVRID DD card to the job steps that are processing IAM data sets, and provide the simple control card image input with the desired parameters. The IAMOVRID DD is normally a DD * that is followed by the control cards within the input job stream. The IAMOVRID could also be a sequential or partitioned data set member that contains the input control cards. The format of the override card for the ACCESS overrides are that they begin with the word ACCESS, which can begin in column one or be preceded by blanks, is followed by one or more blanks, then the override keywords with their values are specified, separated by commas. The keywords and their values may not go beyond column 71. Continuations, if needed, are indicated by leaving a comma after the last keyword on the preceding card, which is followed by other cards with the additional keywords. Full details on the IAM Overrides can be found in IAM-Overrides of the IAM User’s space. If a data set is going to be processed under IAM/RLS, then for the job / address space actually using the data set, the only relevant overrides are RLS, RLSFP, and RLSID, the rest of the overrides are ignored. Shown below are the commonly used IAMACCESS Overrides that are applicable to IAM’s Enhanced Format Files.
Override keywords
Keyword | Description |
|---|---|
BACKUPCOMPRESSED | Indicates that IAMis not to decompress the data when passing the requested records to the requester. The data remains in an IAM data compressed format, which is not usable by application programs for purposes other than copying and reloading an IAM data compressed data set. Backup compressed is only honored on software compressed files, otherwise it is ignored. This option cannot be used with Syncsort. |
BUFSPACE= | Specifies a value, in kilobytes, for that amount maximum amount of buffer space that should be used. Specification of the BUFSPACE override supersedes any MAXBUFNO specification. |
BUF64= | Specifies whether or not IAM acquires buffers in 64-bit addressable virtual storage. Valid values are YES and NO. When 64-bit addressable buffers are being used, IAM acquires buffers in one megabyte increments, and fits as many buffers as it can within each allocated area. The default is based on the Global Options, which as shipped is NO. Use of 64-bit virtual storage for buffers may cause higher CPU times due to the overhead of page fixing that storage area. |
CACHE64 | Specifies keyword with no operands, and that IAM uses a 64-bit virtual storage area to hold an Enhanced format data set’s overflow blocks. The primary benefit is expected for programs accessing the file sequentially, when there are a lot of overflow records. |
DDNAME= | Specifies the DD names of the files to which the override values are applied. The special value &ALLDD can be used to apply the overrides to all files except those that are otherwise explicitly specified. Either DDNAME or DSN is required. |
DSN= | Specifies the name of the data set to which the override values are to be applied. Either DDNAME or DSN is required. |
DYNCORE=nnnn | Specifies the amount of virtual storage, in kilobytes, to be used for IAM’s Dynamic Tabling facility. Storage is acquired from above the line (31-bit addressable) storage within the job’s address space. This keyword cannot be specified if DYNDS is specified. Maximum value is 16000. |
DYNDS=nnnn | Specifies the amount of virtual storage, in megabytes, to be acquired for a Data Space to be used for IAM’s Dynamic Tabling function. Values from 1 to 2048 are permitted. Cannot be specified with the DYNCORE override. |
FRLOG= | Specifies the value to be used for log stream journaling for the file, which will override any FRLOG value specified when the file was defined. This provides users with additional control over log stream journal operations. Valid values are:
|
INDEXSPACE= | Indicates where IAM acquires the virtual storage for the index. Valid values are:
Default value is 64BIT, unless otherwise changed in the IAM Global Options. |
JRNAD= | Specifies the IAM journaling capabilities to be used. When specified on the ACCESS override statement, if the value specified enables journaling, the value will be combined with any specification from the CREATE override when the file was defined or loaded. For files not being processed by IAM/RLS or IAM/PLEX, users must pre-allocate and catalog a log data set to be used for the journaling, which is required to be the name of the IAM data set appended with the characters “.LOG”. IAM journaling will not be active during file loads, reorganizations, or during recovery from the journal. For files being processed by IAM/RLS, the users must specify the journal data sets to the IAM/RLS startup procedure if they want to use journaling, and had not specified to do so when the data set was defined or loaded. The ACCESS override only changes the journaling value for the job step on which it is specified. If the data set is being opened under IAM/RLS, then the JRNAD override MUST be specified under IAM/RLS, and not the batch jobs using IAM/RLS services. Valid values are:
Default value is NONE, unless otherwise specified when the file was defined or loaded. |
LOGSTREAM | Specifies the name of the log stream to contain the log records written based on the FRLOG parameter. This will override the defined log stream name. |
MAXBUFNO=nnnnn | Specifies the maximum number of buffers IAM will use for the specified file. Maximum value allowed is 24576. |
MAXREGION=nnnnn | Specifies the maximum value, in megabytes, which IAM Dynamic Region Adjustment will allow the Extended Private region to be set to. Maximum value allowed is 2048. |
MAXSECONDARY=nn | This applies only to data sets that are not SMS Extended format. Specifies the multiplication factor to be used on the secondary quantity when a data set exceeds 5 extents on a volume. Maximum value that can be specified is 10. |
MINBUFNO=nnnn | Specifies the minimum number of buffers that IAM uses for the specified data set. If specified, this will also be the number of buffers that IAM initially acquires during open. |
NOMEMLIMIT | Specifies that IAM can bypass the memory limit for 64-bit virtual memory when more is needed to expand the overflow index. This is provided, so that users can avoid an outage due to inability to add records to 64-bit virtual memory limit. Primarily intended for long running address spaces such as CICS regions and IAM/RLS or IAM/PLEX. |
NORLS | Specifies thatIAM does not process this file under the IAM RLS address space, even if the installation’s RLS selection criteria indicates that the files are eligible for RLS processing. |
REREAD | Indicates that IAM always reread the overrides each time an IAM file is opened. Normally for batch jobs, IAM will only read the overrides on the first open, and save all the values in virtual storage for subsequent reference. Specifying this keyword allows the overrides to be changed for long running online systems. Reread is the default for CICS, IAM/RLS, and IAM/PLEX address spaces, |
RLS | Indicates that the files are to be processed by the IAM RLS address space. If the IAM RLS address space is not active, the OPEN of the file will fail. |
RLSFP=n | When a file is being sequentially read for non-update and is being processed under IAM RLS, this parameter controls the amount of data that is passed back to the calling address space. Non-zero values offer a sequential fast path function that will reduce the overhead of using IAM RLS. The value specified is a multiplier times the block size of the data set. Specification of 0 will turn off the fast path function, which will guarantee that an application will always get the latest copy of the record. The default is 2, meaning approximately 2 blocks worth of data will be passed back to the calling address space. |
RLSID=nnnn | Specifies the four-character identifier of the IAM/RLS address space, that is to be connected. This keyword is primarily intended to be used with a DD=&ALLDD override card, and then with any other override cards that are specified for that job step. Specification of an RLSID on the ACCESS override does not force a data set to be processed under IAM RLS, but rather directs it to an IAM/RLS address space if IAM RLS processing is otherwise indicated for this data set. |
SHAREOPTION=n | Overrides the VSAM Cross Region Share Option to use for this execution. Provided as the alternative to performing an ALTER on the file to change the Shareoptions. |
TURBO=[YES | NO] | Specifies whether Turbo mode buffering can be used, which indicates that IAM is more aggressive in increasing the number of buffers. Default value is from the IAM Global Options Table, which as shipped is YES. |