A KSDS file stores logical records of a file in fixed length blocks called Control Intervals(CI). In a KSDS Data file, a Control Interval holds several logical records. The logical records within each control-interval are always kept sorted by key-field.
A KSDS File could have thousands of Control Intervals. In a Control Interval, records can be of any size or length. We do not distinguish in particular between fixed-length and variable-length records. However, as a rule, all Control Intervals in KSDS file are exactly equal in size(length).
When a new KSDS file is created, you must specify the size of the Control Intervals in the file. By default, the Control Intervals(CI) in a KSDS File assume a size= 4k(4096) bytes. However, the size of Control Intervals in KSDS Files can lie in the range of 512 bytes <= Control Intervals Size <= 32k
When you create a new KSDS File, the control intervals in it are empty. As you load data into the KSDS file, the Control Intervals are populated with information.
What follows from hereon, shall give you a picture of how Control Intervals look like in Memory.
Control Interval (Very idealistic – Simplified)
Assume that, Control Intervals are 4096 bytes long. A logical record(Employee record) spans 1024 bytes. Then,
No. of records per CI = 4096/1024 = 4 records/CI
Thus, in this example, the Control Interval is completely full(no room for new records).
Control Interval often contains some empty/free space(Close to real model) :
Assume that, Control Intervals are 4096 bytes long. The first logical record = 1000 bytes, the second logical record = 1500 bytes, the third logical record= 1,300 bytes.
Logical Record 1 + Logical Record 2 + Logical Record 3
= 1000 + 500 + 1300 = 2800 bytes.
Thus, the remaining space = 4096 – 2800 = 1,296 bytes is left free. This free-space can be used to accommodate a new logical record. Thus, Control Intervals may also have free-space.
New logical records can be added to a Control Interval, by using the free-space in the Control Interval(CI).
![Image153[1]](http://lh5.ggpht.com/_sQvdFWqMlMg/S4qOTiBgySI/AAAAAAAAClM/am6K17sHwSA/Image153%5B1%5D_thumb%5B2%5D.png?imgmax=800 "Image153[1]")
VSAM Control Interval
![Image152[1]](http://lh3.ggpht.com/_sQvdFWqMlMg/S4qOV1bFULI/AAAAAAAAClU/fGoUC2WU9pE/Image152%5B1%5D_thumb%5B2%5D.png?imgmax=800 "Image152[1]")
Control Interval showing addition of record with key 30
Let's look at the recipe followed by VSAM, to add a new logical record to a KSDS File.
1. VSAM goes through a full-index search to locate the Control Interval(CI) in the KSDS Data file, in which the new record must be placed.
(This search is exactly the same as that used to randomly retrieve a record).
2. After the index search locates the Control Interval(CI), that Control Interval(CI) is loaded into memory. VSAM then searches through the logical records in the Control Interval to determine, where the new record should go.(Recall, that a KSDS file stores all data records in increasing order of the key).
3. The new record is then inserted into the Control Interval(CI), in key sequence, re-arranging the other records, as necessary.
4. The updated Control Interval(CI) is now written back to its original location on the Disk.
Control Interval also contains extra Information(Real Model) :
VSAM treats all the logical records, as if they were variable-length(even if, they are fixed-length). VSAM keeps track of the length of Logical records in a Control Interval, by using special Record-definition Field(RDF), at the end of each Control Interval. This special field that holds the length information for each logical record is 3 bytes long.
Moreover, VSAM also keeps track of the amount of the free-space and its location, within a Control Interval. This meta-information is stored in a special Control Interval-definition Field(CIDF), at the end of each Control Interval. This special field that holds [amount,location] of the free-space for a Control Interval is 4 bytes long.
Control Area(CA) :
A Control Area(CA) is a group of related Control Intervals.
KSDS Files are organised as Control Areas(CA) which in turn contain hundred’s of fixed-length Control Intervals(CI) filled with logical records, free-space and Control information.