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Chapter 5: Working with data sets

Chapter 5: Working with data sets. Chapter objectives. Be able to: Explain what a data set is Describe data set naming conventions and record formats List some access methods for managing data and programs Explain what catalogs and VTOCs are used for

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Chapter 5: Working with data sets

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  1. Chapter 5: Working with data sets

  2. Chapter objectives • Be able to: • Explain what a data set is • Describe data set naming conventions and record formats • List some access methods for managing data and programs • Explain what catalogs and VTOCs are used for • Be able to create, delete, and modify data sets

  3. block size catalog data set high level qualifier (HLQ) library logical record length (LRECL) member PDS and PDSE record format (RECFM) system managed storage (SMS) virtual storage access method (VSAM) VTOC Key terms in this chapter

  4. What is a data set? • A data set is a collection of logically related data records stored on one disk storage volume or a set of volumes. • A data set can be: • a source program • a library of macros • a file of data records used by a processing program. • You can print a data set or display it on a terminal. The logical record is the basic unit of information used by a program running on z/OS.

  5. Dataset Naming

  6. What an access method is • Defines the technique used to store and retrieve data. • Includes system-provided programs and utilities to define and process data sets. • Commonly used access methods include the following: • VSAM: Virtual Storage Access Method • QSAM: Queued Sequential Access Method • BSAM: Basic Sequential Access Method • BDAM: Basic Direct Access Method • BPAM: Basic Partitioned Access Method

  7. DASD: Use and terminology • Direct Access Storage Device (DASD) is another name for a disk drive. • DASD volumes are used for storing data and executable programs. • Data sets in a z/OS system are organized on DASD volumes. • A disk drive contains cylinders • Cylinders contain tracks • Tracks contain data records.

  8. Datasets

  9. Using a data set • To use a data set, you first allocate it. • To set aside (create) space for a new data set on a disk. • To establish a logical link between a job step and any data set • Then, access the data using macros for the access method that you have chosen. • Various ways to allocate a data set: • ISPF data set panel, option 3.2 • Access Method Services • TSO ALLOCATE command • job control language (JCL)

  10. Allocating space on DASD volumes • How space is specified: • explicitly (SPACE parameter) • implicitly (SMS data class) System Management Facilities • Logical records and blocks: • Smallest amount of data to be processed • Grouped in physical records named blocks • Each block has a distinct location and unique address • Data set extents: • Space for a disk data set is assigned in extents • Contiguous number of disk drive tracks (or cylinders) • 128 to 255 extents per volume

  11. F record record record record Fixed records. block block FB record record record record record record Fixed blocked records. BLKSIZE = n * LRECL V record record record Variable records. RDW block block VB record record record record record Variable blocked records. BLKSIZE >= 4 + n * largest LRECL BDW U record record record record Undefined records. No defined internal structure for access method. Record and block descriptors words are each 4 bytes long Data set record formats one physical block on disk is one logical record and all the blocks/records are the same size several logical records are combined into one physical block one logical record as one physical block four-byte Record Descriptor Word (RDW) at the beginning of the record several variable-length logical records (each with an RDW) in one physical block Extra BDW at beginning with total length of block variable-length physical records/blocks with no predefined structure

  12. Types of data sets • We discuss three types in this class: • Sequential, partitioned, and VSAM • A sequential data set is a collection of records written and read in sequential order from beginning to end. • A partitioned data set (PDS) is a collection of sequential data sets, called members. • Consists of a directory and one or more members. • Also called a library. • A PDSE is a partitioned data set extended.

  13. Types of Non-VSAM datasets

  14. PDS versus PDSE • PDS data sets: • Simple and efficient way to organize related groups of sequential files. • PDSE data sets: • Similar to a PDS, but advantages include: • Space reclaimed automatically when a member is deleted • Flexible size • Can be shared • Faster directory searches

  15. What is a data set, and how is it stored

  16. How data is stored in a z/OS system • Data is stored on a direct access storage device (DASD), magnetic tape volume, or optical media. • You can store and retrieve records either directly or sequentially. • You use DASD volumes for storing data and executable programs, including the operating system itself, and for temporary working storage. • You can use one DASD volume for many different data sets, and reallocate or reuse space on the volume.

  17. General Dataset Specifications

  18. Allocating a Dataset in ISPF

  19. How data sets are named • Data set naming convention • Unique name • Maximum 44 characters • Maximum of 22 name segments: level qualifier • The first name in the left: high level qualifier (HLQ) • The last name in the right: low level qualifier (LLQ) • Level qualifiers are separated by '.' • Each level qualifier: • From 1 up to 8 characters • The first must be alphabetical (A-Z) or special (@ # $) • The 7 remaining: alphabetical, national, numeric (0-9) or hyphen (-) • Upper case only • Example: MYID.JCL.FILE2 HLQ: MYID 3 qualifiers • Member name of partitioned data set • 8 bytes long • First byte: alphabetical (A-Z) or special (@ # $) • The 7 remaining: alphabetical, special, numeric (0-9)

  20. Naming Conventions • The letters LIB somewhere in the name indicate that the data set is a library. • The letters PDS are a lesser-used alternative for this. • The letters CNTL, JCL, or JOB somewhere in the name typically indicate the data set contains JCL (but might not be exclusively devoted to JCL). • The letters LOAD, LOADLIB, or LINKLIB in the name indicate that the data set contains executables. (A library with z/OS executable modules must be devoted solely to executable modules.) • The letters PROC, PRC, or PROCLIB indicate a library of JCL procedures.

  21. Naming Conventions • Various combinations are used to indicate source code for a specific language, for example COBOL, Assembler, FORTRAN, PL/I, JAVA, C, or C++. • A portion of a data set name may indicate a specific project, such as PAYROLL. • Using too many qualifiers is considered poor practice. For example, P390A.A.B.C.D.E.F.G.H.I.J.K.L.M.N.O.P.Q.R.S is a valid data set name (upper case, does not exceed 44 bytes, no special characters) but it is not very meaningful. A good practice is for a data set name to contain three or four qualifiers. • Again, the periods count toward the 44-character limit.

  22. Catalogs and VTOCs • z/OS uses a catalog and a volume table of contents (VTOC) on each DASD volume to manage the storage and placement of data sets. • VTOC: • Lists the data sets on a volume • Lists the free space on the volume.

  23. Volume Table of Contents

  24. VTOC

  25. VTOC • Record 1 on the first track • Contains vosler, a 6-char volume serial number • Contains pointer to Volume Table of Content • VTOC • Lists the data sets that reside on the volume • Informatioin about location and size of each data set • Other data set attributes • ICKDSF, standard program, creates label and VTOC • Owner can specify location and size of VTOC

  26. How a catalog is used • A catalog associates a data set with the volume on which the data set is located. • Locating a data set requires: • Data set name • Volume name • Unit (volume device type) • Typical z/OS system includes a master catalog and numerous user catalogs.

  27. stores the full data set name and location of all data sets with a SYS1 prefix Catalog Structure Path to SYS1.A1 data Path to IBMUSER.A1 data

  28. Locating a dataset in MVS

  29. Catalog and Uncataloged Datasets Note the ‘ // ‘ and parm statements used for Job Control Language

  30. Dataset Control Blocks (DSCB)

  31. VTOC Index Structure ISPF option 3.4

  32. Traditional Disk Capacity (DASD)

  33. Large Volume (own device type)

  34. Data management in z/OS • Data management involves all of the following tasks: allocation, placement, monitoring, migration, backup, recall, recovery, and deletion. • Setting aside (allocating) space on DASD volumes • Automatically retrieving cataloged data sets by name • Mounting magnetic tape volumes in the drive • Establishing a logical connection between the application program and the medium • Controlling access to data • Transferring data between the application program and the medium

  35. Data management in z/OS • Storage management is done either manually or through automated processes (or through a combination or both). • In z/OS, Data Facility Storage Management Subsystem (DFSMS) is used to automate storage management for data sets. • Storage Management Subsystem (SMS). • System programmer or storage administrator defines policies that automate the management of storage and hardware devices. • describe data allocation characteristics, performance and availability goals, backup and retention requirements, and storage requirements for the system. • SMS governs these policies for the system and the Interactive Storage Management Facility (ISMF) provides the user interface for defining and maintaining the policies. • The data sets allocated through SMS are called system-managed data sets or SMS-managed data sets.

  36. Data Facility Subsystem Managed Storage (DFSMS) Rules based

  37. VSAM • VSAM is Virtual Storage Access Method • Primarily for applications • VSAM provides more complex functions than other disk access methods • VSAM record formats: • Key Sequence Data Set (KSDS) • Entry Sequence Data Set (ESDS) • Relative Record Data Set (RRDS) • Linear Data Set (LDS)

  38. VSAM Access Method IDCAMS. An IBM program used to process access method services commands. It can be invoked as a job or jobstep, from a TSO terminal, or from within a user's application program.

  39. Simple VSAM control interval Control Interval (CI): logical data area Default Cl size is 4K bytes, max to 32K bytes Contains data records, unused space, Record Descriptor Fields (RDF), and a CI Descriptor Field

  40. VSAM Index Structure

  41. VSAM Keyed Dataset This is the most common use for VSAM. Each record has one or more key fields and a record can be retrieved (or inserted) by key value. This provides random access to data. Records are of variable length.

  42. VSAM Sequential Dataset = ESDS ESDS: Entry Sequence Data Set This form of VSAM keeps records in sequential order. Records can be accessed sequentially. It is used by IMS, DB2, and z/OS UNIX.

  43. VSAM - RRDS This VSAM format allows retrieval of records by number; record 1, record 2, and so forth. This provides random access and assumes the application program has a way to derive the desired record numbers.

  44. VSAM LDS This is, in effect, a byte-stream data set and is the only form of a byte-stream data set in traditional z/OS files (as opposed to z/OS UNIX files). A number of z/OS system functions.

  45. Basic Parms for VSAM dataset Defining a VSAM KSDS using AMS.

  46. z/OS UNIX file systems • z/OS UNIX System Services (z/OS UNIX) allows z/OS to access UNIX files. • A z/OS UNIX file system is hierarchical and byte-oriented. • Files in the UNIX file system are sequential files and are accessed as byte streams. • UNIX files and traditional z/OS data sets can reside on the same DASD volume.

  47. MVS and File Systems

  48. UNIX file system structure

  49. z/OS UNIX file system • A path name identifies a file and consists of directory names and a file name. • A fully qualified file name, which consists of the name of each directory in the path to a file plus the file name itself, can be up to 1023 bytes long. The hierarchical file system allows for file names in mixed case. • The path name is constructed of individual directory names and a file name separated by the forward-slash character, for example: • /dir1/dir2/dir3/myfile

  50. z/OS UNIX file system (cont.) • Like UNIX, z/OS UNIX is case-sensitive for file and directory names. For example, in the same directory, the file MYFILE is a different file than myfile. • The files in the hierarchical file system are sequential files, and are accessed as byte streams. A record concept does not exist with these files other than the structure defined by an application. • The hierarchical file system (HFS) data set which contains the hierarchical file system is a z/OS data set type. • HFS data sets and z/OS data sets can reside on the same DASD volume.

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