Monday, 28 July 2014

COBOL BLock vs Unblock Records.

File is used to store data in form or record. Broadly speaking record are of two type Fixed Length and Variable Length Records.

  • Fixed Length : All of the records in an entire file have the same length.
  • Variable Length : All of the records in an entire file have the variable length.  

Most of the records in files are fixed-length records. That means that all of the records in an entire file have the same length. Occasionally, though, it makes sense to create a file that contains variable-length records. 

That means that a file contains records of different sizes. In most cases, the variable-length records are blocked for the same reasons that fixed-length records are blocked.

Block Records Unblock Records
When records are blocked, two or more records are stored in each sector. Since this makes better use of disk storage, most files contain blocked records.
When records are unblocked, only one record is stored in each sector. 
With blocked records, the disk drive reads or writes a block of records at a time. This means that there’s only one rotational delay for each block of records, and that can improve the efficiency of a program that works with disk files.
With Unblock records, the disk drive reads or write single record at a time. 
The blocking factor is the number of records that are stored in a block. To get the most from blocking, you use a blocking factor that wastes as little disk space as possible. If a block of records spans two or more sectors, all of the sectors are read or written at once.
The COBOL programmer usually doesn’t have to worry about blocking because this is handled automatically by the compiler and operating system. As a result, you write the program as if it were working with unblocked records.

Created with Artisteer

No comments:

Post a comment