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| Multiple Processing: Direct Access |
1. Define cylinder, track and sector.
Cylinder: Cylinder is the collection of tracks of the same numbers on a disk i.e. the tracks that can be read from or written to with a single positioning of the accessing mechanism.
Track: One of a series of concentric rings on the surface of a magnetic disc on which data is recorded.
Sector: In computer disk storage, a sector is a subdivision of a track on a magnetic disk or optical disc. Each sector stores a fixed amount of user-accessible data.
2. What does it mean by indexed file?
A file from which records may be retrieved directly by means of one or more indexes; can be accessed sequential as well.
3. What does it mean by ISAM?
ISAM means Indexed Sequential Access Method. It is a means of organizing record on a direct access storage device that provides for both direct and sequential.
4. What does mean by IOCS?
A part of the operating system of a computer that handles the reading and writing of records.
5. How to update an ISAM file?
To update an ISAM file, the programmer must supply IOCS with the key of the record needed and then issue a read command. When the read command is executed, IOCS retrieves from the field the key that it uses as a search argument in searching the indexes and the prime data area.
6. What does it mean by adding a record to an ISAM file?
The key of the record is moved to the record key field, and the command to write an additional record is given. IOCS will set the I/O switch to zero unless a record with that key already exists in the file; in that case the I/O switch is set to one.
More information:
Adding record to an ISAM file may be batch or online.
If batch is used Access may be direct or sequential. Normally used direct access.
7. What does mean by VSAM file?
A means of organizing records on a direct –access storage device that provides for bath direct and sequential access.
VSAM file is capable of handling both fixed-length records and variable length-records.
Fixed length-records: Records which require differing amount of storage is called fixed length-records.
Variable-length records: The record which may change in over time is called fixed-length records.
8. What is random file?
A file which records appear to be in particular order because the location of a record is determined by manipulations performed on the key of the record (hashing)
9. What is hashing algorithm?
The location or address at which a record is stored in a random file is determined by manipulations performed on the record. The manipulation are expressed as a hashing algorithm.
10. What is index set?
In a VDSM file, the index that contains the highest key in each control area; also includes any higher-level indexes that may be required.
11. What is sequence set?
In a VISAM file the lowest level index is called the sequence set.
It contains-
i. The highest key in each control interval within the control area &
ii. The starting location of each control interval.
12. What are full index files?
An index in order by record key that contains the key and location of every record in the file. The entry contains the record key and the address on the disk of the record.
Note: The records do not need to be stored in sequence and the may be fixed variable in length.
More information:
The index is order by record key.
A fully indexed file needs on overflow area, nor does it needs to reserve free space to add records.
Creating a fully indexed file:
Create fully indexed file may be in any order.
As record is stored on the disk, an entry is made in the index to record to both in the key and the location of the record.
When all records have been stored, the index is sorted.
13. What is DASD?
DASD means Direct-Access Storage Device. The most common example of DASD is magnetic disc.
14. What is master index?
In an ISAM file, the highest-level index. Master index is an optional index that contains the highest key in each track of the cylinder index.
15. What is direct access?
The ability to retrieve a specific record from a file without having to first read all preceding records.
16. Define the following Keywords:
• Control Area: In a VASM file, control area is a collection of control intervals.
• Control Interval: In a VASM file, the unit of storage that is transferred on a read or write operation.
• Cylinder: The collection of tracks of the same number on a disk, that is, the tracks that can be read or written to with a single positioning of the accessing mechanism.
• Cylinder Index: In an ISAM file, the tracks set aside in a cylinder to provide for records that overflow from the cylinder’s prime data area when additions are made to the data.
• Cylinder Overflow Area : In an ISAM file, the tracks set aside in a cylinder to provide for records that overflow from the cylinder’s prime data area when additions are made to the file.
• Direct Access: The ability to go directly to a record without having to first read all preceding records.
• DASD: DASD stands for Direct Access Storage Device. The most common example of a DASD is magnetic disk.
• File reorganization : Creating a new version of a direct-access file to eliminate inefficiency in storage utilization and/or processing as a result of the addition and/or deletion of records.
• Fixed length record: A record that will never change in size.
• Free space: In a VSAM file, space set aside for the adding of records.
• Full index: An index in order by record key that contains the key and location of every record in a file.
• Hashing: Deriving the location for a record in a random file from the record key.
• Independent overflow area: In an ISAM file, one or more cylinders that are used for records that have overflowed from the prime data area when there is no longer any room left in the appropriate cylinder overflow area.
• Index set: In a VSAM file, the index that contains the highest key in each control area; also includes any higher-level indexes that may be required.
• Indexed file: A file from which records may be retrieved directly by means of one or moiré indexes; can be accessed sequentially as well.
• Indexed sequential-access method (ISAM): A means of organizing records on a direct-access storage device that provides for both direct and sequential access.
• IOCS: A part of the operating system of a computer that handles the reading and writing of records.
• Master Index: In an ISAM file, the highest-level index; an optional index that contains the highest key in each track of the cylinder index.
• Online real-time: A system, such as a reservation system, in which the output from processing a transaction is received immediately and affects the activity taking place.
• Prime data area: In an ISAM file, the area in which records are placed when the file is created.
• Random file: A way to represent instructions that uses ordinary English; a planning tool for structured programming.
• Relative track number: A way of identifying the location of a track for a file on disk by determining how far it is from the first track used for the file.
• Sequence set: In a VSAM file, the lowest-level index. Consists of a record for each control area containing the highest key in each control interval in the control area; allows for the sequential processing of the file.
• Spaced sequential search: A Searching method in which table or index entries at fixed intervals are examined until either the desired entry is found or it can be determined that the desired entry has been passed over. In the latter case, the entries between the last two entries examined are then searched serially.
• Track: One of a series of concentric rings on the surface of a magnetic disk on which data is recorded.
• Track index: In an ISAM file, track index is the lowest-level index. It is located on track 0 of each cylinder for the file. The track index contains the highest key on each prime data track in the cylinder as well as pointers to records that have overflowed from those tracks.
• Variable length record: A record which may change in size over the time because of a change in the number of fields in the records. It is a change in the size of fields or both.
• VSAM (Virtual-storage access method): A means of organizing records on a direct-access storage device that provides for both direct and sequential access.
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