Ch02 Storage Media

8/4/2019 Ch02 Storage Media

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SistemSistem Basis DataBasis Data

CSCS33343334

Storage Media

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Fakultas Informatika ITTelkom2010


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Disks, Memory, Files

Query Optimizationand Execution

Relational Operators

The BIG picture

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Files and Access Methods

Buffer Management

Disk Space Management

DB


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Disks and Files

DBMS stores information on disks.

In an electronic world, disks are a mechanicalanachronism!

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READ: transfer data from disk to main memory(RAM).

WRITE: transfer data from RAM to disk. Both are high-cost operations, relative to in-memory

operations, so must be planned carefully!

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Data Storage

Deal with very large amounts of dataefficiently. How does a computer system store and

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What representations and data structures bestsupport efficient manipulations of the data?


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Why Not Store Everything in Main

Memory?

Costs too much.

Main memory is volatile.

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.

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File System

Why not using file system?

Can be used to store data information for a long period as well.Consistency and failure recovery capability

Problems:

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no query language, no efficient access for a data itemsupport of the creation of database is limitedconsistency control is not adequate

Management of Data is complex


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Smaller, FasterPrimary Storage/Main Memory

Internal storage For currenly used data

Cache, main memory

The Storage Hierarchy

www.ittelkom.ac.idSource: Operating Systems Concepts 5th Edition

Bigger, Slower

econ ary s orage Disk for the main database Electronic Disk, Magnetic disk

Tertiary Storage Tapes for archiving older

Versions of the data

Optical Disk, Magnetic Tape

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Storage Classification

Volatile storage

Loses contents when power is switched off Cache, Main memory

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Non-volatile storage Contents persist even when power is switched off Includes secondary and tertiary storage

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Primary Storage

Cache fastest and most costly form of storage; volatile; managed

by the computer system hardware.

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Main memory: fast access generally too small (or too expensive) to store the entire

database

Volatile contents of main memory are usually lost if apower failure or system crash occurs.

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Primary Storage

Input Storage Area

To store data input Program Storage area

Store instructions to mana e data

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Working Storage Area To process data

Output Storage Area

To store data temporally before output to user

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Primary Storage

CONTROL UNIT SECTION

INPUT

STORAGE

AREA

PROGRAM

STORAGE AREA OUTPUT

STORAGE

AREA

CONTROL UNIT SECTION

PROGRAM

STORAGE AREA

OUTPUTINPUT

PRIMARY

STORAGE SECTION


ARITHMETIKA LOGICAL UNIT SECTION

WORKING

STORAGE AREA

STORAGEAREA

WORKING

STORAGE AREA

ARITHMETIKA LOGICAL UNIT SECTION

STORAGEAREA

Control unit section, Primary storage section, ALU section

Is part of CPU.


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Electric Disk Non-Volatile

Data can be written at a location only once, but location can be erasedand written to again

Widely used in embedded devices such as digital cameras

Is a type of EEPROM (Electrically Erasable Programmable Read-OnlyMemory)

Secondary Storage

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Magnetic-disk Data is stored on spinning disk, read/written magnetically

Data must be moved from disk to main memory for access, and writtenback for storage

direct-access possible to read data on disk in any order,

Much larger capacity

Survives power failures and system crashes

disk failure can destroy data, but is rare

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Terms Used in the Hardware

Description

Sector - A segment or arc of a track.

Block - is the division of a track into equal sized portionsby the operating system. Interblock Gaps - These are fixed sized segments that

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separate t e oc s. Read/Write Head - Actual reads/writes the information tothe disk.

Cylinder - Tracks with the same diameter that are locatedon the disk surface of a disk pack.

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Tertiary Storage

Optical storage

non-volatile, data is read optically from a spinning disk using a laser CD-ROM and DVD

Write-one, read-many (WORM) optical disks used for archivalstorage (CD-R, DVD-R, DVD+R)

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- , - , ,

and DVD-RAM)

Tape storage non-volatile, used primarily for backup (to recover from disk failure),

and for archival data sequential-access much slower than disk

very high capacity

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Components of a Disk

The platters spin

The arm assembly ismoved in or out toposition a head on a

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es re rac .

Tracks under heads

make a cylinder

Only one head reads/writes

at any one time.

Block sizeis a multipleof sector size

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Accessing a Disk Page

Time to access (read/write) a disk block:

seek time(moving arms to position disk head on track) rotational delay(waiting for block to rotate under head) transfer time(actually moving data to/from disk surface)

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.

Seek time varies between about 0.3 and 10msec Rotational delay varies from 0 to 4msec Transfer rate around .08msec per 8K block

Key to lower I/O cost: reduce seek/rotation delays!Hardware vs. software solutions?

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Arranging Pages on Disk

`Next block concept:

blocks on same track, followed by blocks on same cylinder, followed by blocks on adjacent cylinder

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on disk (by `next), to minimize seek androtational delay.

For a sequential scan, pre-fetchingseveral pagesat a time is a big win!

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Performance Measures of Disks

Access timethe time it takes from when a read or write request is issued to when data transferbegins.

Seek time time it takes to reposition the arm over the correct track. Rotational latency time it takes for the sector to be accessed to appear under

the head.

-

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the rate at which data can be retrieved from or stored to the disk. 25 to 100 MB per second max rate, lower for inner tracks Multiple disks may share a controller, so rate that controller can handle is also

important E.g. ATA-5: 66 MB/sec, SATA: 150 MB/sec, Ultra 320 SCSI: 320 MB/s

Fiber Channel (FC2Gb): 256 MB/s Transfer Rate (tr)

The rate at which information can be transferred to or from the disk.

Block Transfer Time (btt)

The time it takes to transfer the data once the read/write head has been positioned.18


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Disk Parameters

parameter typical value source

T track size 50 KB fixedB block size 1 KB formatted

s seek time 10 msec fixed

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p rotational velocity 1,000 rps fixedrd (average) rotational delay 3 msec .5*(1/p)

tr transfer rate 1MB/sec T*pbtt block transfer time 1 msec B/trrbtt random block transfer time 13 msec s + rd + bttcbtt contiguous block transfer time 1 msec B/tr


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Parallelizing Disk Access Using RAID

RAID - Stands for Redundant Arrays of InexpensiveDisks or Redundant Arrays of Independent Disks.

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RAIDs are used to provide increased reliability,increased performance or both.

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RAID Levels

Level 0 - has no redundancy and the best writeperformance but its read performance is not as good as

level 1.

Level 1 - uses mirrored disks which provide redundancyand improved read performance.



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RAID Levels

Level 2 - provides redundancy using Hamming Codes

Level 3 - uses a single parity disk.



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RAID Levels

Level 4 and 5 - use block-level data striping with level 5distributing the data across all the disks.

Level 6 - uses the P + Q redundancy scheme makinguse of the Reed-Soloman codes to protect against thefailure of 2 Disks.



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TERIMA KASIH

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Ch02 Storage Media