Disk Storage & Files in Database Systems: Storage Devices, Records, & File Operations - Pr, Study notes of Deductive Database Systems

Download Disk Storage & Files in Database Systems: Storage Devices, Records, & File Operations - Pr and more Study notes Deductive Database Systems in PDF only on Docsity!

Chapter 13

Disk Storage, Basic File Structures, and

Hashing. Copyright © 2004 Pearson Education, Inc.

Copyright © 2004 Ramez Elmasri and Shamkant Navathe

Disk Storage Devices (cont.) Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition

z

Preferred secondary storage device for highstorage capacity and low cost.

z

Data stored as magnetized areas on magneticdisk surfaces.

z

A

disk pack

contains several magnetic disks

connected to a rotating spindle.

z

Disks are divided into concentric circular^ tracks

on each disk

surface

. Track capacities

vary typically from 4 to 50 Kbytes.

Copyright © 2004 Ramez Elmasri and Shamkant Navathe

Disk Storage Devices (cont.) Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition

Because a track usually contains a largeamount of information, it is divided intosmaller

blocks

or

sectors

z

The division of a track into

sectors

is hard-coded on

the disk surface and cannot be changed. One type ofsector organization calls a portion of a track thatsubtends a fixed angle at the center as a sector. z

A track is divided into

blocks

. The block size B is

fixed for each system. Typical block sizes range fromB=512 bytes to B=4096 bytes. Whole blocks aretransferred between disk and main memory forprocessing.

Copyright © 2004 Ramez Elmasri and Shamkant Navathe

Disk Storage Devices (cont.) Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition

z^

A^

read-write

head moves to the track that contains the block to

be transferred. Disk rotation moves the block under the read-write head for reading or writing. z^

A physical disk block (hardware) address consists of a cylindernumber (imaginery collection of tracks of same radius from allrecoreded surfaces), the track number or surface number (withinthe cylinder), and block number (within track). z^

Reading or writing a disk block is time consuming because of theseek time s and rotational delay (latency)

rd

.

z^

Double buffering can be used to speed up the transfer ofcontiguous disk blocks.

Copyright © 2004 Ramez Elmasri and Shamkant Navathe

Disk Storage Devices (cont.) Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition

Copyright © 2004 Ramez Elmasri and Shamkant Navathe

Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition

Records

z

Fixed and variable length records z

Records contain fields which have values of a particulartype (e.g., amount, date, time, age) z

Fields themselves may be fixed length or variable length z

Variable length fields can be mixed into one record:separator characters or length fields are needed so that therecord can be “parsed”.

Copyright © 2004 Ramez Elmasri and Shamkant Navathe

Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition

Blocking

z

Blocking: refers to storing a number of records in one block on the disk. z

Blocking factor (

bfr

) refers to the number of records per

block. z

There may be empty space in a block if an integralnumber of records do not fit in one block. z

Spanned Records

: refer to records that exceed the size of

one or more blocks and hence span a number of blocks.

Chapter 13-

Copyright © 2004 Ramez Elmasri and Shamkant Navathe

Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition

Files of Records (cont.)

z

File records can be

unspanned

(no record can span two

blocks) or

spanned

(a record can be stored in more than

one block). z

The physical disk blocks that are allocated to hold therecords of a file can be

contiguous

,^ linked

, or

indexed

z

In a file of fixed-length records, all records have thesame format. Usually, unspanned blocking is used withsuch files. z

Files of variable-length records require additionalinformation to be stored in each record, such as separator characters

and

field types

. Usually spanned

blocking is used with such files.

Copyright © 2004 Ramez Elmasri and Shamkant Navathe

Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition

Operation on Files

Typical file operations include: z

OPEN:

Readies the file for access, and associates a

pointer that will refer to a

current

file record at each

point in time. z

FIND:

Searches for the first file record that satisfies a

certain condition, and makes it the current file record. z

FINDNEXT:

Searches for the next file record (from

the current record) that satisfies a certain condition, andmakes it the current file record. z

READ:

Reads the current file record into a program

variable. z

INSERT:

Inserts a new record into the file, and makes

it the current file record.

Copyright © 2004 Ramez Elmasri and Shamkant Navathe

Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition

Unordered Files

z

Also called a

heap

or a

pile

file.

z

New records are inserted at the end of the file. z

To search for a record, a

linear search

through the file

records is necessary. This requires reading andsearching half the file blocks on the average, and ishence quite expensive. z

Record insertion is quite efficient. z

Reading the records in order of a particular fieldrequires sorting the file records.

Copyright © 2004 Ramez Elmasri and Shamkant Navathe

Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition

Ordered Files

z

Also called a

sequential file

z

File records are kept sorted by the values of an

ordering

field

z

Insertion is expensive: records must be inserted in the correct order

. It is common to keep a separate unordered

overflow

(or

transaction

) file for new records to

improve insertion efficiency; this is periodically mergedwith the main ordered file. z

A

binary search

can be used to search for a record on its

ordering field value

. This requires reading and searching

log

of the file blocks on the average, an improvement 2 over linear search. z

Reading the records in order of the ordering field is quiteefficient.

Copyright © 2004 Ramez Elmasri and Shamkant Navathe

Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition

Average Access Times

The following table shows the average access time to

access a specific record for a given type of file

Copyright © 2004 Ramez Elmasri and Shamkant Navathe

Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition

Hashed Files

z^

Hashing for disk files is called

External Hashing

z^

The file blocks are divided into M equal-sized

buckets

, numbered

bucket

, bucket 0

, ..., bucket 1

M-

. Typically, a bucket corresponds to

one (or a fixed number of) disk block. z^

One of the file fields is designated to be the hash key of the file. z^

The record with hash key value K is stored in bucket i, wherei=h(K), and h is the

hashing function

.

z^

Search is very efficient on the hash key. z^

Collisions occur when a new record hashes to a bucket that isalready full. An overflow file is kept for storing such records.Overflow records that hash to each bucket can be linked together.