File Access Methods

Determines How CA IDMS/DB Gains Access to Files
idms
Determines How CA IDMS/DB Gains Access to Files
When an application program issues a call to CA IDMS/DB for retrieval or storage of a record or row of data, CA IDMS/DB maps the database page that contains the record or row to the corresponding block or blocks in the file. The means by which this mapping occurs varies according to the access method in use:
  • EXCP (z/OS, z/VSE)
  • SAM (z/VSE)
  • DASD Block I/O (z/VM)
  • VSAM (z/OS, z/VSE)
EXCP Access Method
The EXCP access method is used in z/OS and z/VSE to take advantage of extended addressing. Using EXCP as an access method, CA IDMS/DB maps the database page number to a relative track and record number. The database page size must equal the block size of the file.
SAM Access Method
Using SAM as an access method, CA IDMS/DB maps the first database page number to a relative block number (RBN) within the sequential access file. It then reads forward sequentially from that RBN. The database page size must equal the block size of the file.
DASD Block I/O
In z/VM, all CA IDMS/DB files are allocated as separate minidisks and are accessed using DASD Block I/O.
For more information, see the
Installing section -- z/VM
.
VSAM Access Method
CA IDMS/DB can take advantage of extended addressing when accessing data by means of the VSAM access method. All VSAM macros use the AMODE=31 and RMODE=31 parameters. Therefore, all VSAM control blocks are allocated above the 16-megabyte line.
Accessing VSAM Database Files
Using VSAM as an access method to VSAM database files, CA IDMS/DB maps the database page number to a VSAM control interval and issues a request to VSAM for that control interval.
Accessing Native VSAM Files
Existing VSAM files to be accessed by CA IDMS/DB are referred to as
native VSAM files
because they are not formatted into pages as is the case with all other file types. CA IDMS/DB accesses native VSAM files using VSAM record-level services. A native VSAM file can have one of the following structures:
  • Key-sequenced (KSDS)
  • Entry-sequenced (ESDS)
  • Relative record (RRDS)
Regardless of the type of file being accessed, each is represented by a single record type described to CA IDMS/DB in a non-SQL schema definition.
For more information, see 17.6, “Considerations for Native VSAM Files".
Choosing Between VSAM and Non-VSAM File Types
In z/OS and z/VSE, you may define database files as either VSAM or non-VSAM.
z/VSE:
To define non-VSAM files on FBA disk devices (type 3310 or type 3370), use a sequential label (that is, an SD attribute on the DLBL statement).