Dataset attributes including LRECL (Logical Record Length), RECFM (Record Format), and BLKSIZE (Block Size) fundamentally affect how datasets are stored, accessed, and displayed. These attributes directly impact your editing and viewing experience in ISPF, determining line lengths, formatting, and editing constraints. This tutorial covers these attributes in detail and explains how they affect editing and viewing operations.
Understanding dataset attributes is crucial for effective editing, as they determine what you can do, how data is displayed, and what constraints you must work within. Knowing how attributes affect editing helps you work efficiently and avoid errors.
Understanding Dataset Attributes
Dataset attributes define the physical and logical structure of datasets.
Key Attributes
Essential dataset attributes include:
LRECL: Logical Record Length
RECFM: Record Format
BLKSIZE: Block Size
DSORG: Data Set Organization
Other attributes (space, volume, etc.)
Why Attributes Matter
Attributes matter because:
They determine dataset structure
They affect how data is stored
They impact editing and viewing
They cannot be changed after creation
They define editing constraints
LRECL (Logical Record Length)
LRECL specifies the length of each logical record in bytes.
What is LRECL?
LRECL:
Stands for Logical Record Length
Specifies record length in bytes
Is measured in bytes (characters)
Determines maximum (or fixed) line length
Is a critical attribute for editing
LRECL for Fixed Format
For fixed-format files (RECFM=F or FB):
LRECL specifies the exact length of every record
All records must be exactly LRECL bytes
Short lines are padded with spaces
Long lines are truncated to LRECL
You cannot exceed LRECL when editing
LRECL for Variable Format
For variable-format files (RECFM=V or VB):
LRECL specifies the maximum record length
Records can be shorter than LRECL
Records cannot exceed LRECL
Each record can have a different length
You can edit up to LRECL maximum
LRECL Impact on Editing
LRECL affects editing by:
Setting the maximum line length
Determining whether lines are padded
Controlling text truncation
Defining editing boundaries
Affecting column positioning
Common LRECL Values
Common LRECL values and their uses:
80: Standard for source code (COBOL, JCL, etc.)
132: Wide format for listings and reports
256: Common for data files
32760: Maximum for variable format
Other values based on specific needs
RECFM (Record Format)
RECFM specifies how records are organized and affects display and editing.
What is RECFM?
RECFM:
Stands for Record Format
Specifies how records are organized
Determines whether records are fixed or variable length
Indicates whether records are blocked
Affects how the editor displays and handles files
Common RECFM Values
Common RECFM values:
F: Fixed (unblocked) - all records same length
FB: Fixed Blocked - all records same length, grouped in blocks
V: Variable (unblocked) - records can vary in length
VB: Variable Blocked - records can vary, grouped in blocks
U: Undefined - no record structure
RECFM Impact on Editing
RECFM affects editing by:
Determining whether lines must be fixed length
Controlling whether lines can vary in length
Affecting how lines are displayed
Influencing padding and truncation behavior
Determining editing constraints
Fixed Format Editing
When editing fixed-format files:
All lines appear the same length
Short lines are padded with spaces
You cannot exceed LRECL
Text beyond LRECL is truncated
BOUNDS helps enforce limits
Variable Format Editing
When editing variable-format files:
Lines can have different lengths
Each line uses only the space it needs
You can edit up to LRECL maximum
Lines are not padded
More flexible editing
BLKSIZE (Block Size)
BLKSIZE affects storage efficiency but has indirect impact on editing.
What is BLKSIZE?
BLKSIZE:
Stands for Block Size
Specifies the size of physical blocks on disk
Is measured in bytes
Affects storage efficiency
Does not directly affect editing display
BLKSIZE Characteristics
BLKSIZE:
Is typically calculated from LRECL and RECFM
Affects I/O performance
Influences storage efficiency
Does not change what you see when editing
Is set when the dataset is created
BLKSIZE Calculation
BLKSIZE is typically:
Calculated as a multiple of LRECL
Optimized for the storage device
Set to maximize efficiency
Not directly editable
BLKSIZE Impact
BLKSIZE impacts:
Storage efficiency (larger blocks are more efficient)
I/O performance (larger blocks can be faster)
Does not affect line length or editing constraints
Does not change display in the editor
How Attributes Affect Editing
Dataset attributes directly impact your editing experience.
Line Length Constraints
Attributes determine:
Maximum line length (LRECL)
Whether lines must be fixed length (RECFM)
Whether padding occurs (fixed format)
Whether truncation occurs (exceeding LRECL)
Display Behavior
Attributes affect display:
Fixed format shows all lines same length
Variable format shows natural line lengths
Padding is visible in fixed format
Column positioning differs by format
Editing Constraints
Attributes create constraints:
Fixed format requires staying within LRECL
Variable format allows up to LRECL maximum
Text beyond LRECL is lost or prevented
BOUNDS helps enforce limits
Viewing Dataset Attributes
You can view dataset attributes in ISPF.
In the Editor
In the editor, you can see:
RECFM and LRECL in the status line
Dataset attributes in file information
Format information in editor displays
Using Data Set Utility
To view full attributes:
Use Option 3.2 (Data Set Utility)
Select the dataset
View dataset information
See LRECL, RECFM, BLKSIZE, and other attributes
Editing Implications
Understanding how attributes affect editing helps you work effectively.
Fixed Format Editing
When editing fixed-format files:
All lines must be exactly LRECL bytes
Short lines are automatically padded
You cannot type beyond LRECL
Use BOUNDS to enforce limits
Column positioning is critical
Variable Format Editing
When editing variable-format files:
Lines can vary in length
Each line uses only needed space
You can edit up to LRECL maximum
Lines are not padded
More flexible editing
Exceeding LRECL
What happens when you exceed LRECL:
Fixed format: Text is truncated or prevented
Variable format: Text beyond LRECL is truncated
Editor may display warnings
Data beyond LRECL is lost
Always stay within LRECL
Viewing Implications
Attributes also affect how files are viewed.
Fixed Format Viewing
When viewing fixed-format files:
All lines appear the same length
Padding spaces are visible
Column alignment is consistent
Structure is uniform
Variable Format Viewing
When viewing variable-format files:
Lines appear at their natural lengths
No padding is visible
Column alignment may vary
Structure is flexible
Best Practices
Following best practices helps you work effectively with dataset attributes:
Know Your Attributes: Always check LRECL and RECFM before editing
Respect LRECL: Stay within LRECL limits when editing
Use BOUNDS for Fixed: Use BOUNDS to enforce limits for fixed-format files
Understand Padding: Be aware that fixed-format files pad short lines
Check Attributes First: View attributes before starting to edit
Plan for Attributes: Consider attributes when creating new datasets
Verify After Editing: Verify your edits respect attribute constraints
Understand Format Impact: Know how RECFM affects editing behavior
Common Scenarios
Here are common scenarios involving dataset attributes.
Editing COBOL Source
For COBOL source (typically RECFM=FB, LRECL=80):
All lines are exactly 80 bytes
Short lines are padded to 80
You must stay within column 72 (typical COBOL limit)
Use BOUNDS 1 72 to enforce limits
Column positioning is critical
Editing Data Files
For data files (may vary by format):
Fixed format: Maintain exact record length
Variable format: Lines can vary up to maximum
Respect LRECL limits
Understand format requirements
Explain Like I'm 5: Dataset Attributes
Think of dataset attributes like rules for how your notebook pages work:
LRECL is like saying "each line on your paper can only be this long." If you say "80 characters," then every line must be 80 characters—if you write less, you fill the rest with spaces, and if you write more, it gets cut off. It's like having a ruler that says "you can only write this far on each line"!
RECFM is like the type of paper you're using. Fixed format is like having lined paper where every line is exactly the same length. Variable format is like having blank paper where you can write lines of different lengths, as long as no line is longer than the maximum. It's like choosing what kind of paper to use!
BLKSIZE is like how the pages are organized in your notebook. It doesn't change how long your lines can be, but it affects how efficiently the notebook stores your writing. It's like having pages that are organized in a way that makes the notebook work better, but you don't really see it when you're writing!
So dataset attributes are like rules that tell your "notebook" (dataset) how long lines can be, whether they all have to be the same length, and how everything is organized on the "pages" (blocks)!
Practice Exercises
Complete these exercises to reinforce your understanding of dataset attributes:
Exercise 1: Viewing Attributes
Practice viewing: use Data Set Utility to view LRECL, RECFM, and BLKSIZE of different datasets, check attributes in the editor status line, and understand attribute information. Build familiarity with attributes.
Exercise 2: Editing Fixed Format
Practice fixed format: edit fixed-format files, observe how all lines are the same length, understand padding, practice staying within LRECL, and learn fixed-format editing constraints. Master fixed-format editing.
Exercise 3: Editing Variable Format
Practice variable format: edit variable-format files, observe how lines can vary in length, understand variable format behavior, practice editing up to LRECL maximum, and learn variable-format editing. Master variable-format editing.
Exercise 4: LRECL Limits
Practice LRECL limits: work with files of different LRECL values, observe how LRECL affects editing, test what happens when exceeding LRECL, and understand LRECL constraints. Master LRECL concepts.
Exercise 5: Attribute Impact
Practice understanding impact: compare editing fixed vs variable format files, observe how attributes affect display and editing, understand attribute implications, and learn to work effectively with different attributes. Build comprehensive understanding.