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Data Base Organization
and File Structure
Suresh Choubey, Ph.D.
July 7, 2015
Data Base Organization and File
Structure: Introduction
1
File Structure

Storage hierarchy of computer storage media ?

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primary storage  main + cache
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Fast access but limited storage capacity
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volatile storage
secondary storage magnetic disks & tapes
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Large capacity ,lower cost but slower access

non – volatile storage
Data Base Organization and File
Structure: Introduction
2
File Structure

Storage hierarchy of computer storage media ?

Data bases store large amount of data that must
persist over long periods of time
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Magnetic tapes are off line – used for backup

enter record gap of 0.6
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sequential access
Magnetic disks are on-line and data is organized as
files of records
July 7, 2015
Data Base Organization and File
Structure: Introduction
3
File Structure

H/W address of a block = combination of surface
number, track # & block #
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fixed head disk - # of read/write heads are fixed and
are equal to the number of tracks
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movable head disk – and acutuator is used
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ransfer time = seek time + block transfer time
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random access device
Data Base Organization and File
Structure: Introduction
4
File Structure
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Buffering of blocks
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most useful when processes can run concurrently in a simultaneous
fashion (multiple processors may enlist or a special I/O processor)
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Double buffering
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CPU can start processing a block once its transfer to main memory
is completed, while at the same time the disk I/O processor reads
and transfers the next block into a different buffer
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saves seek time and rotational delay
Data Base Organization and File
Structure: Introduction
5
File Structure
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Record Types:
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Each record consists of related data values or items
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Each value corresponds to a field of record and takes a few bytes
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Records type or record format: a collection of fields with their data
type
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A file is a sequence of records
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File of fixed – length records: If every record in file has the same size (in bytes)
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File of variable – length records: different sizes
Data Base Organization and File
Structure: Introduction
6
File Structure

Why variable length Records?
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File records are of same type but fields are of variable size (variable
length fields) e.g. variable length name
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File records are of same type but one or more fields may have
multiple value for individual records
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such fields are called repeating field and a group of values for field is
called repeating
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July 7, 2015
File records are of same type but one or more fields are optional
File contains records of different record types and hence of varying
size (mixed file)
Data Base Organization and File
Structure: Introduction
7
File Structure

Storing the Records:
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Files of fixed length records: field locations w.r.t.
beginning of records is known
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Files of variable length records: (a) use separator
character, (b) each record as a set of < field name,
field value > pairs, (c) < field type, field value > pair
July 7, 2015
Data Base Organization and File
Structure: Introduction
8
File Structure

Record Blocking: Spanned vs Unspanned Records
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Let B = block size
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R = record size of file of fixed-length records
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Let there exists a R, then bfr = [(B/R)], where bfr =
blocking factor for the file.
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Here unused space = B – (bfr * R) bytes.
Data Base Organization and File
Structure: Introduction
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File Structure

Spanned organization: to utilize this unused space,
a record can be stored in next block or in some
other block (using a pointer to point to that block)
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Here record is spanned over more than one block.

used with variable length record

should be used if record size is large, otherwise wastage
will be too high
July 7, 2015
Data Base Organization and File
Structure: Introduction
10
File Structure

Unspanned organization: if we do not allow
records to cross block boundaries

used with fixed length record
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simplifies record processing as every record starts at
one location
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here bfr is average # of records per block.
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# of blocks required = [r/bfr] blocks
Data Base Organization and File
Structure: Introduction
11
File Structure

Advantage of blocking
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Blocking reduces #of data transfer operations
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Blocking reduces # of interblock gaps
Data Base Organization and File
Structure: Introduction
12
File Structure

Allocating file blocks on disks
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Contiguous allocation: efile blocks are allocated to consecutive disk
blocks.
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Advantage: fast reading of whole file using double buffering
Disadvantage: file cannot be expanded.
Linked allocation: each file block contains a pointer to the next
file block
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Advantage: file expansion is possible
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Disadvantage: slow reading of who file
Data Base Organization and File
Structure: Introduction
13
File Structure

Allocating file blocks on disks
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Clusters: clusters of consecutive disk blocks and clusters
are linked.
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-- combination of above two
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-- clusters are also called segments or extents
Indexed allocation: one or more index blocks contain
pointers to the actual file blocks
July 7, 2015
Data Base Organization and File
Structure: Introduction
14
File Structure

File descriptions or file headers

Contains information to determine disk address of the file
blocks as well as record format descriptions (such as field
length) order of fields within a record for fixed length
unspanned records and field type code, separator
characters and record type codes for variable–length
records
July 7, 2015
Data Base Organization and File
Structure: Introduction
15
File Structure

Operations on File
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Find (or locate)
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Read (or Get)
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Find Next (record at a time operation)
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Delete
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Modify
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Insert
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Find all
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Find ordered – set at a time operations
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Reorganize
July 7, 2015
Data Base Organization and File
Structure: Introduction
16
File Structure

File Organizations
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Refers to organization of data of a file into records , blocks, and
access structures.

Access Methods
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Consists of a group of programs that allow operations as above

File may be static (update operations are rarely performed).

volatile (update information are a
Data Base Organization and File
Structure: Introduction
17

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