IAM overrides for IAM data set definition

There are various unique attributes for IAM data sets that can optionally be set when the data set is defined through the use of the IAM Override facility. The IAM override facility is provided because there is no mechanism within the DEFINE CLUSTER command to pass this information. Included among the attributes that can be specified are IAM Data Compression, a hardware compression dictionary, and secondary space options. Most of these attributes are based on defaults from the IAM Global Options Table, which are normally set up during the IAM product installation. With appropriate choices made during installation, there should be a very infrequent need to use the IAM Overrides during the file define. An overview of the IAM Override facility is presented here, while complete information is provided in IAM-Overrides of the space.

Related topic

IAM Override Statement Examples

IAMOVRID DD statement

The use of the IAM Override facility is triggered by providing an IAMOVRID DD card in the JCL, which usually consists of in-stream card image input (that is, //IAMOVRID DD *). This DD can also reference a card image sequential file on DASD, or a member of a PDS. Each control card contains a word indicating the type of override (that is, CREATE or ACCESS), then after a blank is followed by one or more operands. For a file definition, use the CREATE override statement. The CREATE override requires either the DD= or the DSN= operand, which identifies the data set to which the override is applicable. A value of DD=&ALLDD will apply the override to any IAM data set that is not explicitly overridden.

The IAM Override facility can be used for the Define Cluster commands issued under IDCAMS or TSO. It does not work for defines done through JCL DD cards or the TSO ALLOC command. Various IAM overrides can be specified directly on the IAMISPF panel. The IAM Override process will most likely work with other software products that can Define VSAM files. Examples of using the IAM Override facility for file definition are included in with the IDCAMS Define Cluster examples later in this section.

CREATE override operands

The following list contains the CREATE Override keywords which are applicable to the file definition process. For the most part, the CREATE Override keywords have the same meaning and implication when used during the file load, as they do during the file define. The underscored portion of the keywords indicates their minimum abbreviation. The keywords applicable to the define process are described in the following table:

CREATE Override Operands

Keyword

Description

BLKSIZE=nn

Specifies the block factor (1 through15) or block size of the IAM data set. The default value is based on the specified CI Size and maximum record size. Typically the default is the maximum block size that can be used to obtain 4 blocks per track.

BUFSPACE=

Specifies the maximum amount of storage in kilobytes (1024) that can be used for buffers. If specified other than 0, the value will supersede any MAXBUFNO override. This value is saved in the data characteristics and will be used to compute MAXBUFNO when the data set is opened for processing. MAXBFNO is computed as:

(BUFSPACE *1024)/BLOCKSIZE

Default is no buffer space will be permanently associated with the data set.

DATACOMPRESS=

Indicates whether IAM is to data compress this data set. Valid values are:

  • YES—Indicates that the data set will have its data compressed, using the default data compression technique from the IAM Global Options Table.
  • NO—Indicates that no data compression will be used for this data set.
  • HW—Indicates IAM will use the z/OS hardware compression instruction to compress the data. If the DICT= override specifying a compression dictionary name is not specified, IAM will dynamically build a compression dictionary when the file is loaded, based on the data in the first several records.
  • HWE—Specifies that IAM will utilize an enhanced dictionary build process that increases the amount of potential data strings that are eligible for compression. Use of this process is likely to improve the amount of compression, however, based on the data in the file results will vary. Files that use this feature are compatible with all prior IAM releases that support hardware compression.
  • SW—Indicates that IAM will use its standard software compression algorithm. No dictionaries are used with this form of compression.

The default is based on the IAM Global Options Table, which as shipped is any data set that is 75 tracks (5 cylinders) or larger is eligible for IAM data compression. The default technique is also based on the IAM Global Options Table, which is shipped as software compression.

DDNAME=

Specifies the data set to which the override is applied. When used during a DEFINE process, the value specified MUST match the value specified for the FILE parameter. If this value is set to &ALLDD, then the overrides are applicable to any IAM file that is not otherwise explicitly overridden.

DICTIONARY=

Specifies the four character suffix for the name of the user provided hardware compression dictionary. The dictionary must be in load module format, with the first four characters being ‘IAMD’. Review the section on Hardware Compression for information on creating and naming the compression dictionary.

The default is that IAM will create a dictionary during the file load process using data from the initial records written.

DSN=

Specifies the name of the data set to which the override is applied. Either DDNAME= or DSN= must be specified.

DSNTYPE=

Specifies the type of data set to be used for the IAM file. Valid values are:

  • LARGE—Specifies that the IAM data set will use the Large Format Sequential data set type, which can exceed 64K tracks per volume and is not required to reside on DFSMS managed volumes.
  • EXT—Specifies that the IAM data set will use the SMS Extended format. These data sets can exceed 64K tracks per volume and must reside on DFSMS managed volumes.Use of Extended Format for IAM data sets is not recommended.
  • BASIC—The IAM data set will be a basic non-VSAM data set, which is limited to a maximum of 64K tracks per volume.

Default value is LARGE for IAM Enhanced format data sets and BASIC for IAM Compatible format data sets, unless the DFSMS Data Class indicates SMS Extended format and the Global Option ENABLE=SMSEXTOK is set.

ENHANCED

Specifies that IAM is to create an Enhanced format IAM data set. This type of data set uses a dynamic overflow area, which can acquire additional DASD extents during file updates, as needed. The default is taken from the IAM Global Options Table, which is shipped as Enhanced format and is the recommended IAM file format.

INTEGRATED=nn

Specifies CI% free space, especially useful for data compressed ESDS data sets.

JRNAD=

For Enhanced format files, specifies the IAM journaling capabilities to be used.

For files not being processed by IAM/RLS or IAM/PLEX, or using IAM/RLS journaling, users must 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 / cluster, 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 or journaled by IAM/RLS or IAM/PLEX, the users must specify the journal data sets to the IAM/RLS or IAM/PLEX startup procedure if they want to use journaling, and had not specified to do so when the data set was defined or loaded.

Valid values are:

  • BOTH or ALL—The IAM log data set will contain both before and after images. This will enable the user to perform either a forward recovery, or a backward (backout) type of recovery.
  • BEFORE—The IAM log data set will contain before (undo) images of updated records. This option allows backward (backout) recoveries only.
  • AFTER—The IAM log data set will contain after (redo) images of updated records. This option allows forward recoveries only.
  • NONE—(Default) The IAM journaling feature will not be used for this IAM data set.

MAXBUFNO=nnnn

Specifies the maximum number of buffers to be used during file access for this data set. A maximum of 24576 can be specified.

MAXSECONDARY=nn

Specifies a value from 0 through 25 as a multiplication factor for the secondary space quantity, when the data set exceeds five extents on a volume. When overridden on the DEFINE, this will be the default for both file load and file access for this data set. MAXSECONDARY is ignored on DFSMS Extended Format files due to the large number of extents that they can take per volume.

If the IAM global option SPACEADJ is set to Enabled and the SMSEXTOK option is set to Disabled, then the initial IAM define data set allocation will have the primary and secondary values multiplied by this MAXSECONDARY value. No five extents volume adjustment will be done later. See the Space Adjustment subsection earlier in this section for additional details.

MINBUFNO=nnn

Specifies the minimum number of buffers to be used during file access for this data set.

MULTIVOLUME=
[PRIMARY | SECONDARY]

Specifies space quantity to request when IAM believes that the next extent will be placed on the next candidate volume. The default is specified in the IAM Global Options Table, and as shipped is MULTIVOLUME=PRIMARY.

PRO=[YES | NO]

Specifies whether or not IAM will use the Prime Related Overflow (PRO) for this data set. This format is only recommended for use on data sets that will have close to a million or more records in overflow and if shared, must be used under IAM/RLS or IAM/PLEX. Valid values are:

  • YES—Use the PRO format for this data set.
  • NO—Do not use the PRO format for this data set.

Default value is based on the IAM Global Options, which is shipped as NO.

PSEUDORBA

For ESDS files, indicates that the file can exceed 4 GB of user data. IAM generates RBA values that are different than normal VSAM values. This attribute cannot be used with ESDS files used by software that depends on the normal VSAM values.

RELEASE=[YES|NO]

Indicates whether or not IAM is to release unused and unreserved DASD space whenever the file is loaded. Default is to release space on the first load only, and when a secondary space value is specified, because z/OS 1.11 release will not work if DSORG=DA has been specified as the default Global Option.

RLSID[=nnnn]

When specified with no operand, indicates that this data set is to be processed using IAM/RLS, regardless of Share Options or the IAMRLS Data Set Name Tables. The operand, if specified, is the four-character identifier of the preferred IAMRLS address space that this data set is to be processed by.

UNIT=

Specifies the unit name to specify when allocating the IAM data set. The default is SYSDA for non-specific volume allocations, or the generic unit type, like 3390.

VAROVERFLOW=
[YES|NO]

For Enhanced format files, indicates that IAM can use true variable length records in the Extended Overflow area. Variable Overflow provides a more efficient use of DASD space. The default is from the IAM Global Options table, which is shipped set to YES.

XESDS | NOXESDS

For ESDS files, XESDS specifies to use an 8-byte RBA value so the ESDS can exceed 4 GB of user data. The application programs must be able to handle the 8-byte RBA. This is consistent with the VSAM Extended Addressability support. NOXESDS indicates that a 4-byte RBA will be used that will limit the size to 4 GB.

Default is based on IAM Global Options which as shipped is XESDS is disabled so that IAM will use a 4-byte RBA.

 

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