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Database Management Systems Unit 3

Sikkim Manipal University Page No.: 34

Unit 3 Record Storage and Primary File

Organization

Structure

3.1 Introduction

Objectives

Self Assessment Question(s) (SAQs)

3.2 Secondary Storage Devices


3.3 Buffering of BlocksSelf Assessment Question(s) (SAQs)

3.4 Placing File Records on Disk


3.5 Operation on Files


3.6 Files of Unordered Records (Heap Files)


3.7 Files of Ordered RecordsSelf Assessment Question(s) (SAQs)

3.8 Hashing Techniques


3.9 Summary

3.10 Terminal Questions (TQs)

3.11 Multiple Choice Questions (MCQs)

3.12 Answers to SAQs, TQs, and MCQs

3.12.1 Answers to Self Assessment Questions (SAQs)

3.12.2 Answers to Terminal Questions (TQs)

3.12.3 Answers to Multiple Choice Questions (MCQs)

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3.1 Introduction

The collection of data is stored on some computer storage medium. TheDBMS software can then retrieve update and process this data as needed.

Computer storage media includes two main categories:

Primary Storage: This category includes, storage media that can be

operated directly by the computer central processing unit [CPU], such as

the computer main memory and smaller but faster cache memories.

Secondary storage devices include magnetic disks, optical disks, tapes

and drums, and usually are of larger capacity, cost less and provide

slower access to data than primary storage devices. Data in secondary

storage cannot be processed directly by the CPU; it must first be copied

into primary storage.

Database typically stores large amounts of data that must exist over long

periods of time. Most databases are stored permanently on magnetic disk

(secondary storage) for the following reasons:

Generally, databases are too large to fit entirely in main memory.

Secondary storage devices are nonvolatile storage, whereas main

memory is often called volatile storage.

The cost of storage per unit of data is less for disk than for primary

storage.

Objectives

To know about

Secondary Storage Devices

Buffering of Blocks

Placing File Records on Disk

Operation on Files

Files of Unordered Records (Heap Files)

Files of Ordered Records

Hashing Techniques

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Self Assessment Question(s) (SAQs) (for section 3.1)

1. Write on the categories used in computer storage media.

3.2 Secondary Storage Devices

This section describes some characteristics of magnetic disk and magnetic

tape storage devices.

Hardware Description of Disk Devices:

Magnetic disks are used for storing large amounts of data. The most basic

unit of data on the disk is a single bit of information. By magnetizing an area

on disk in certain ways, we can represent a bit value of either 0 [zero] or

1 [one]. The capacity of a disk is the number of bytes it can store, usually in

kilobytes [Kbytes or 1000 bytes] megabytes and gigabytes [Gbyte or 1

billion bytes]. Disk capacities continue to grow as technology improves.

Disks are all made of magnetic material shaped as a thin circular disk and

protected by plastic or acrylic cover. A disk is single sided if its stores

information on only one of its surfaces, and double-sided if both surfaces

are used. To increase storage capacity, disks are assembled into a disk

pack which may include as many as 30 surfaces. Information is stored on

disk surface on concentric circles, each having a distinct diameter. Each

circle is called a track. For disk packs, the tracks with the same diameter on

the various surfaces are called a cylinder. The concept of a cylinder is

important, because data stores on the same cylinder can be retrieved much

faster than if it distributed among different cylinders. Track usually contains

a large amount of information; it is divided into smaller blocks or sectors.

The division of a track into equal sized blocks or pages is set by the

operating system during formatting.

A disk is called a random access addressable device. Transfer of data

between main memory and disk takes place in units of blocks. The

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hardware address of a block is a combination of a surface number, track

number and block number.

The actual hardware mechanism that reads or writes a block is the disk

read/write head, which is part of a system called a disk drive.

A read/write head includes an electronic component attached to a

mechanical arm. Disk packs with multiple surfaces are controlled by several

read/write heads one for each surface.

The disk drive begins to rotate the disk whenever a particular read or write

request is initiated. Once the read/write head is positioned on the righttrack, and the block specified in the block address moves under the

read/write head, the electronic component of the read/write head is

activated to transfer the data.

To transfer a disk block, given its address, the disk drive must first

mechanically position the read/write head on the correct track. The time

required to do this is called the seek time. Following that, there is another

delay - called the rotational delay or latency while the beginning of the

desired block rotates into position under the read/write head. Some

additional time is needed to transfer the data; this is called the block transfer

time. Hence, the total time needed to locate and transfer an arbitrary block,

given its address, is the sum of the seek time, rotational delay, and block

transfer time. The seek time and rotational delay are usually much larger

than the block transfer time.

Magnetic Tape Storage Devices:

Magnetic tapes are sequential access devices; to access the n th block on

tape, we must first read the proceeding n 1 blocks. Data is stored on reels

of high capacity magnetic tape, somewhat similar to audio or videotapes.

A tape drive is required to read the data from or write the data to a tape reel.

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Tape access can be slow, and tapes are not used to store on-line data.

However, tapes serve a very important function that of backing up the

database. One reason for backup is to keep copies of disk files in case the

data is lost because of disk crash, which can happen if the disk read/write

head touches the disk surface because of mechanical malfunction. For this

reason disk files are periodically copied to tape. Tapes can also be used to

store excessively large database files.

Self Assessment Question(s) (SAQs) (for section 3.2)

1. Explain different types of secondary storage.

3.3 Buffering of Blocks

Before processing any data, data in terms of blocks is copied into the main

memory buffer. When several blocks need to be transferred from disk to

main memory, several buffers can be reserved in the main memory to speed

up the transfer. While one buffer is being read or written, with the help of

Input/Output processor, the CPU can process data in the other buffer.

The figure below illustrates how two processes can proceed in parallel.

Process A and B are running concurrently in an interleaved fashion.

Buffering is most useful when processes can run concurrently in a

simultaneous fashion by both 1/0 processor and CPU. The CPU can start

processing a block once the data is transferred to main memory from

secondary memory, at the same time the disk 1/0 processor can be reading

and transferring the next block into a different buffer. This technique is

called double buffering and can also be used to write a continuous stream of

blocks from memory to the disk. Double buffering permits continuous

reading or writing of data on consecutive disk blocks, which eliminates the

seek time and rotational delay for all but the first block transfer. Moreover,

data is kept ready for processing, thus reducing the waiting time in the

programs.

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Figure 3.1: Interleaved concurrency versus parallel execution.

Figure 3.2: Use of two buffers, A and B, for reading from disk.

Self Assessment Question(s) (SAQs) (For Section 3.3)

1. Explain the concept of buffering blocks.

3.4 Placing file records on disk

Record Types: Data is usually stored in the form of records. Each record

consists of a collection of related data values. Records usually describe

entities and their attributes. For example, an EMPLOYEE record represents

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an employee entity, and each field value in the record specifies some

attribute of that employee, such as NAME, BIRTHDATE, SALARY orSUPERVISOR. A collection of field names and their corresponding data

types constitutes a record type or record format definition.

Files, Fixed-length Records, and Variable-length Records: A file is a

sequence of records.

Fixed length records:

All records in a file are of the same record type. If every record in the file

has exactly the same size [in bytes], the file is said to be made up of

fixed length records.

1 Modern Database Management MCFadden

2 Database Solutions Connolly

Variable length records:

If different records in the file have different sizes, the file is said to be made

up of variable length records. The variable length field is a field whose

maximum allowed length would be specified. When the actual length of the

value is less than the maximum length, the field will take only the required

space. In the case of fixed length fields, even if the actual value is less than

the specified length, the remaining length will be filled with spaces of null

values.

A file may have variable-length records for several reasons:

Records having variable length fields:

The file records are of the same record type, but one or more of the

fields are of varying size. For example, the NAME field of EMPLOYEE

can be a variable-length field.

1 Modern Database Management MCFadden

2 Database Solutions Connolly

Records having Repeating fields

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fields may have multiple values for individual records. Group of values

for the field is called repeating group.

1 Modern DatabaseManagement

3 MCFadden Hoffer Prescott

2 Database Solutions 2 Connolly Begg

Here the record length varies depending on the number of authors.

Records having optional fields:


fields are optional. That is some of the fields will not have values in all

the records. For example there are 25 fields in a record, and out of 25 if

10 fields are optional, there will be a wastage of memory. So only the

values that are present in each record will be stored.

Record Blocking and Spanned versus Unspanned Records:

The records of a file must be allocated to disk blocks. If the block size is

larger than the record size, each block will contain numerous records.

Some files may have unusually large record sizes that cannot fit in one

block. Suppose that the block size is B bytes. For a file of fixed lengthrecords of size R bytes, with B R we can fit bfr = [(B/R)] records per block.

The value bfr is called the blocking factor for the file. In general R may not

divide B exactly, so we have some unused space n each block equal to

R (bfr *R) bytes.

To utilize this unused space, we can store part of the record on one block

and the rest on another block. A pointer at the end of the first block points

to the block containing the remainder of the record. This organization is

called spanned, because records can span more than one block. Whenever

a record is larger than a block, we must use a spanned organization. If

records are not allowed to cross block boundaries, the organization is called

unspanned. This is used with fixed-length records having B R.

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We can use bfr to calculate the number of blocks b needed for file of r

records:

b = [(r/bfr)} blocks.

Figure 3.3: Types of record organization (a) Unspanned (b) Spanned

Allocating File Blocks on Disk:

There are several standard techniques for allocating the blocks of a file on

disk. In contiguous (sequential) allocation the file blocks are allocated to

consecutive disk blocks. This makes reading the whole file very fast, using

double buffering, but it makes expanding the file difficult. In linked allocation

each file block contains a pointer to the next file block. A combination of the

two allocates clusters of consecutive disk blocks, and the clusters are linked

together.. Clusters are sometimes called segments or extents.

File Headers: A file header or file descriptor contains information about a

file, that is needed by the header and includes information to determine the

disk addresses of the file blocks as well as to record format descriptions,

which may include field lengths and order of fields within a record for fixed-

length unspanned records and field type codes, separator characters.

To search for a record on disk, one or more blocks are copied into main

memory buffers. Programs then search for the desired record utilizing the

information in the file header. If the address of the block that contains the

desired record is not known, the search programs must do a linear search

through the file blocks. Each file block is copied into a buffer and searched

until either the record is located. This can be very time consuming for a

large file.

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1. Explain the method of placing file records on disk

3.5 Operations on f iles

Operations on files are usually grouped into retrieval operations and update

operations such as insertion or deletion of records or by modification of field

values.

Find (or locate): Searches for the first record satisfying search

condition.

Read (or Get): copies the current record from the buffer to a program

variable.

Find Next: Searches for the next record.

Delete: Deletes the current record

Modify: Modifies some field values for the current record.

Insert: Inserts a new record into the file.


1. Explain different file operations.

3.6 Files of Unordered Records (Heap Files)

In the simplest and most basic type of organization, records are placed in

the file in the order in which they are inserted, and new records are inserted

at the end of the file. Such an organization is called a heap or pile file.

Inserting a new record is very efficient: the last disk block of the file is copied

into a buffer; the new record is added; and the block is then rewritten back

to the disk. However, searching for a record using linear search is an

expensive procedure.

To delete a record, a program must first find it, copy the block into a buffer,

then delete the record from the buffer, and finally rewrite the block back to

the disk. This leaves extra unused space in the disk block. Another

technique used for record deletion is a marker stored with each record. A

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record is deleted by setting the deletion marker to a certain value. A different

value of the marker indicates a valid (not deleted) record. Search programs

consider only valid records in a block when conducting their search. Both of

these deletion techniques require periodic reorganization of the file. During

reorganization, records are packed by removing deleted records.


1. Explain the concept of unordered files.

3.7 Files of ordered records [sorted files]:

We can physically order the records of a file on disk based on the values of

one of their fields-called the ordering field. If the ordering field is also a key

field of the file, a field guaranteed to have a unique value in each record,

then the field is also called the ordering key for the file.

Ordered records have some advantages over unordered files. First, reading

the records in order of the ordering field values becomes extremely efficient,

since no sorting is required. Second, finding the next record in an ordering

field usually requires no additional block accesses, because the next recordis in the same block as the current one [unless the current record is the last

one in the block]. Third, using a search condition based on the value of an

ordering key field results in faster access when the binary search technique

is used.

NAME SSN BIRTH-DATE JOB SALARY SEX

Block 1 Aaron, Ed

Abbott Daine

:

Acosta, Marc

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Block 3 Adams, John

Adams, Robin :

Akers,Jan

Block 3 Alexander, Ed

Alfred, Bob

:

Allen Sam

Block 4 Allen, Troy

Anders, Kach

:

Anders Rob

Block 5 Anderson, Zach

Angeli, Joe

:

Archer, Sue

Block 6 Arnold, Mack

Arnold, Steven

:

Atikins, Timothy

:

Block n-1 Wong, James

Wood, Donald

:

Woods, Manny

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Block n Wright, Pam

Wyatt, Charles:

Zimmer, Byron

Figure 3.4: Some blocks of an ordered (unspanned) file or EMPLOYEE

records wi th NAME as the ordering key field.

Inserting and deleting records are expensive operations for an ordered file

because the records must remain physically ordered. To insert a new

record; we must find its correct position in the file, based on its ordering field

value, and then make space in the file to insert the record in that position.

For a large file this can be very time consuming. For record deletion the

problem is less severe if we use deletion markers and reorganize the file

periodically.


1. Explain the concept of sorted files.

3.8 Hashing TechniquesOne disadvantage of sequential file organization is that we must use linear

search or binary search to locate the desired record and that results in more

I/O operations. In this there are a number of unnecessary comparisons. In

hashing technique or direct file organization, the key value is converted into

an address by performing some arithmetic manipulation on the key value,

which provides very fast access to records.

Key Value Hash function Address

Let us consider a hash function h that maps the key value k to the value

h(k). The VALUE h(k) is used as an address.

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The basic terms associated with the hashing techniques are:

1) Hash table: It is simply an array that is having address of records.

2) Hash function: It is the transformation of a key into the corresponding

location or address in the hash table (it can be defined as a function that

takes key as input and transforms it into a hash table index).

3) Hash key: Let 'R' be a record and its key hashes into a key value called

hash key.

Internal Hashing

For internal files, hash table is an array of records, having array in the range

from 0 to M-1. Let as consider a hash function H(K) such that H(K)=key mod

M which produces a remainder between 0 and M-1 depending on the value

of key. This value is then used for the record address. The problem with

most hashing function is that they do not guarantee that distinct value will

hash to distinct address, a situation that occurs when two non-identical keys

are hashed into the same location.

For example: let us assume that there are two non-identical keys k1=342

and k2=352 and we have some mechanism to convert key values to

address. Then the simple hashing function is:

h(k) = k mod 10

Here h(k) produces a bucket address.

To insert a record with key value k, we must have its key first. Eg: Consider

h(K-1) =K1% 10 will get 2 as the hash value. The record with key value 342

is placed at the location 2, another record with 352 as its key value

produces the same has address i.e. h(k1) = h(k2). When we try to place the

record at the location where the record with key K1 is already stored, thereoccurs a collision. The process of finding another position is called collision

resolution. There are numerous methods for collision resolution.

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1) Open addressing: With open addressing we resolve the hash clash by

inserting the record in the next available free or empty location in the

table.

2) Chaining: Various overflow locations are kept, a pointer field is added to

each record and the pointer is set to address of that overflow location.

External Hashing for Disk Files

Figure 3.5: Matching bucket numbers to disk block addresses

Handling Overflow for Buckets by Chaining

Hashing for disk files is called external hashing. Disk storage is divided into

buckets, each of which holds multiple records. A bucket is either one disk

block or a cluster of continuous blocks.

The hashing function maps a key into a relative bucket number. A table

maintained in the file header converts the bucket number into the

corresponding disk block address.

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Figure 3.6: Handling overflow fo r buckets by chaining

The collision problem is less severe with buckets, because many records

will fit in a same bucket. When a bucket is filled to capacity and we try to

insert a new record into the same bucket, a collision is caused. However, we

can maintain a pointer in each bucket to address overflow records.

The hashing scheme described is called static hashing, because a fixed

number of buckets 'M' is allocated. This can be serious drawback for

dynamic files. Suppose M be a number of buckets, m be the maximum

number of records that can fit in one bucket, then at most m*M records will

fit in the allocated space. If the records are fewer than m*M numbers,

collisions will occur and retrieval will be slowed down.

Dynamic Hashing Technique

A major drawback of the static hashing is that address space is fixed.

Hence it is difficult to expand or shrink the file dynamically.

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In dynamic hashing, the access structure is built on the binary

representation of the hash value. In this, the number of buckets is not fixed

[as in regular hashing] but grows or diminishes as needed. The file can start

with a single bucket, once that bucket is full, and a new record is inserted,

the bucket overflows and is slit into two buckets. The records are distributed

among the two buckets based on the value of the first [leftmost] bit of their

hash values. Records whose hash values start with a 0 bit are stored in one

bucket, and those whose hash values start with a 1 bit are stored in another

bucket. At this point, a binary tree structure called a directory is built. The

directory has two types of nodes.

1. Internal nodes: Guide the search, each has a left pointer corresponding

to a 0 bit, and a right pointer corresponding to a 1 bit.

2. Leaf nodes: It holds a pointer to a bucket a bucket address.

Each leaf node holds a bucket address. If a bucket overflows, for example:

a new record is inserted into the bucket for records whose hash values start

with 10 and causes overflow, then all records whose hash value starts with

100 are placed in the first split bucket, and the second bucket contains

those whose hash value starts with 101. The levels of a binary tree can be

expanded dynamically.

Extendible Hashing: In extendible hashing the stored file has a directory

or index table or hash table associated with it. The index table consists of a

header containing a value d called the global depth of the table, and a list of

2d pointers [pointers to data block]. Here d is the number of left most bits

currently being used to address the index table. The left most d bits of a

key, when interpreted as a number give the bucket address in which the

desired records are stored.

Each bucket also has a header giving the local depth d1.

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Of that bucket specifies the number bits on which the bucket contents are

based. Suppose d=3 and that the first pointer in the table [the 000 pointer]

points to a bucket for which the local depth d1 is 2, the local depth 2 means

that in this case the bucket contains all the records whose search keys start

with 000 and 001 [because first two bits are 00].

To insert a record with search value k, if there is room in the bucket, we

insert the record in the bucket. If the bucket is full, we must split the bucket

and redistribute the current records plus the new one.

For ex: To illustrate the operation of insertion using account file. We

assume that a bucket can hold only two records.

Brighton 100

Downtown 200

Downtown 300

Mianus 400

Perryridge 500

Perryridge 600

Perryridge 700

Redwood 800

Round Bill 900

Sample account file

Hash function for branch name

Branch-name H(branch-name)

Brighton 0010 1101

Downtown 1010 0011

Mianus 1100 0001

Perryridge 1111 0001

Redwi=ood 0011 0101Round Hill 1101 1000

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Let us insert the record (Brighton, 100). The hash table (address table)

contains a pointer to the one-bucket, and the record is inserted. The second

record is also placed in the same bucket (bucket size is 2).

When we attempt to insert the next records (downtown, 300), the bucket is

full. We need to increase the number of bits that we use from the hash

value i.e., d=1, 21=2 buckets. This increases entries in the hash address

table. Now the hash table contains two entries i.e., it points to two buckets.

The first bucket contains the records whose search key has a hash value

that begins with 0, and the second bucket contains records whose search

key has a hash value beginning with 1. Now the local depth of bucket =1.

Next we insert (Mianus, 400). Since the first bit of h(Mianus) is 1, the new

record should place into the 2nd bucket, but we find that the bucket is full.

We increase the number of bits for comparison, that we use from the hash

to 2(d=2). This increases the number of entries in the hash table to 4 (22 =

4). The records will be distributed among two buckets. Since the bucket that

has prefix 0 was not split, hash prefix 00 and 01 both point to this bucket.

Figure 3.7: Extendable Hash Structure

Next (perryridge, 500) record is inserted into the same bucket as Mianus.

Next insertion of (Perryridge, 600) results in a bucket overflow, causes an

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increase in the number of bits (increase global depth d by 1 i,e d=3), and

thus increases the hash table entries. (Now the hash table has 23

= 8

entries).

Figure 3.8: Extendable Hash Structure

The records will be distributed among two buckets; the first contains all

records whose hash value start with 110, and the second all those whose

hash value start with 111.

Advantages of dynamic hashing:

1. The main advantage is that splitting causes minor reorganization, since

only the records in one bucket are redistributed to the two new buckets.

2. The space overhead of the directory table is negligible.

3. The main advantage of extendable hashing is that performance does not

degrade as the file grows. The main space saving of hashing is that no

buckets need to be reserved for future growth; rather buckets can be

allocated dynamically.

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Disadvantages:

1. The index tables grow rapidly and too large to fit in main memory. When

part of the index table is stored on secondary storage, it requires extra

access.

2. The directory must be searched before accessing the bucket, resulting

in two-block access instead of one in static hashing.

3. A disadvantage of extendable hashing is that it involves an additional

level of indirection.


1. What is the disadvantage of sequential file organization? How do you

overcome it?

2. Describe different hashing techniques.

3. How do you handle bucket overflow? Explain.

4. Explain the concept of dynamic Hashing Technique.

5. What are the advantages and disadvantages of Dynamic Hashing?

3.9 Summary

In this unit we learnt concepts such as Secondary Storage Devices,

Buffering of Blocks, Placing File Records on Disk, Operation on Files, Files

of Unordered Records (Heap Files), Files of Ordered Records and Hashing

Techniques.

3.10 Terminal Questions (TQs)

1. Define storage medium, differentiate between primary and secondary

Storage.

2. Describe the characteristics of magnetic disk and magnetic tape storagedevices.

3. Discuss the concept of variable length records.

4. Write down all the basic terms associated with the hashing techniques

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3.11 Multip le Choice Questions (MCQs)

1. Transfer of data between main memory and disk takes place in.(a) units of blocks

(b) units of bytes

(c) units of MB

(d) None of the above

2. is the time required to position the read/write head on the correct

track.

(a) Seek time

(b) response time

(c) throughput


3. . are sequential access devices

(a) Magnetic tapes

(b) CDs

(c) Pen drives


4 In. the space overhead of the directory table is negligible.

(a) Static Hashing(b) Dynamic Hashing

(c) Extendible Hashing


3.12 Answers to SAQs, TQs, and MCQs

3.12.1 Answers to Self Assessment Quest ions (SAQs)

For Section 3.1

1. Computer storage media includes two main categories:

Primary Storage: This category includes storage media that can be

operated directly by the computer central processing unit [CPU],

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such as the computer main memory and smaller but faster cache

memories.

Secondary storage devices include magnetic disks, optical disks,

tapes and drums and usually are of larger capacity cost less and

provide slower access to data than primary storage devices. Data in

secondary storage cannot be processed directly by the CPU; it must

first be copied into primary storage. (Refer section 3.1)

For Section 3.2

1. Magnetic disk and magnetic tape storage devices (Refer section 3.2)

For Section 3.3

1. Buffering of blocks:

Before processing any data, data in terms of blocks are copied into main

memory buffer. When several blocks need to be transferred from disk to

main memory, several buffers can be reserved in main memory to speed

up the transfer. While one buffer is being read or written, with the help

of Input/Output processor, the CPU can process data in the other buffer.

(Refer section 3.3)

For Section 3.4

1. Placing file records on disk:

Record Types: Data is usually stored in the form of records. Each record

consists of a collection of related data values. (Refer section 3.4)

For Section 3.5

1. Operations on files:Operations on files are usually grouped into retrieval operations and

update operations such as insertion or deletion of records or by

modification of field values. (Refer section 3.5)

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For Section 3.6

1. Files of Unordered Records (Heap Files)In the simplest and most basic type of organization, records are placed

in the file in the order in which they are inserted, and new records are

inserted at the end of the file. Such an organization is called a heap or

pile file. (Refer section 3.6)

For Section 3.7

1. Files of ordered records [sorted files]:

We can physically order the records of a file on disk based on the values

of one of their fields-called the ordering field. If the ordering field is alsoa key field of the file a field guaranteed to have a unique value in each

record, then the field is also called the ordering key for the file. (Refer

section 3.7)

For Section 3.8

1. One disadvantage of sequential file organization is that we must use

linear search or binary search to locate the desired record and that

results in more I/O operations. In this there is a number of unnecessary

comparisons. In hashing technique or direct file organization, the key

value is converted into an address by performing some arithmetic

manipulation on the key value, which provides very fast access to

records. (Refer section 3.8)

2. Internal Hashing, External Hashing etc (Refer section 3.8)

3. Handling Overflow for Buckets is done by Chaining (Refer section 3.8)

4. In dynamic hashing, the access structure is built on the binary

representation of the hash value. In this, the number of buckets is not

fixed [as in regular hashing] but grows or diminishes as needed. (Refer

section 3.8)

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3.12.2 Answers to Terminal Questions (TQs)

1. The collection of data stores on some computer storage medium.

(Refer section 1.1)

2. Hardware Description of Disk Devices: Magnetic disks are used for

storing large amounts of data. The most basic unit of data on the disk is

a single bit of information. By magnetizing an area on disk in certain

ways, we can represent a bit value of either 0 [zero] or 1 [one]. (Refer

section 3.2)

3. Variable length records: If different records in the file have different

sizes, the file is said to be made up of variable length records. (Refer

section 3.4)

4. The basic terms associated with the hashing techniques are:

i) Hash table: It is simply an array that is having address of records.

ii) Hash function: It is the transformation of a key into the

corresponding location or address in the hash table, (it can be

defined as a function that takes key as input and transforms it into a

hash table index)

iii) Hash key: Let 'R' be a record and its key hashes into a key valuecalled hash key. (Refer section 3.8)

3.12.3 Answers to Multiple Choice Questions (MCQs)

1. A

2. B

3. A

4. B

03-unit3

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