Ekternal Memory

Magnetik Disk Magnetic Disk

Disk is a circular disc made of metal or plastic is coated with a substance that can dimagnetisasi. The data recorded on them and can be read from the disk by using coils pengkonduksi (conducting coil) is called head disk. In the writing operation, the electric current in the disk memagnetisasi head. the magnetic field on the disc that is moving under the head produce an electric current to the head. During the reading and writing operations, head is stationary while moving the dish underneath.



Data Organization and Formatting
Organization of data on the disc-shaped ring of concentric rings called tracks. Each track width equal to the width of head. Track the adjacent separated by gaps. Gap aims to prevent / reduce errors due to slipping of head or magnetic field interference. density (density), in bits per linear inch, the next track in the higher (more tightly) than the outer tracks. Data is stored in block-sized area known as the sector. There is usually between 10 and 100 sectors per track. Sector-adjacent sector separated by a gap-gap and intra-or inter-track record.
Layout disk data include:
• Track
• Inter-Track Gaps
• Density (density)

One example is the disk formatting disk track format Winchester (Seagate ST506)
• Each track contains 30 sector fixed length
• Each track contains 600 bytes
• Each sector holds 512 bytes of data plus control information useful to the disk controller.




Characteristic
There are several characteristics of Disk Systems:
• Movement head
- Fixed head the disk was gotten by one head read/wrote per track was still having several head
read/wrote per surface. All head was placed to arms memanjang all through track.
- Movable head the disk was only gotten by one head read/wrote per surface. Arms where head
was placed could memanjang and became shorter to head towards some track.
• Portabilitas the Disk
Disk was to a disk drive that consisted of arms, the stem that could move the disk, and electronics
equipment for the need input and output the binary data.
- non-removable is the disk permanently was to the disk drive.
- Removable the disk is could be released and replaced with the other disk.
• The surface that dimagnetisasi
- Double-sided is the two sides of his surface dimagnetisasi.
- Single-sided is only one surface that dimagnetisasi (the disk bermuka single).
• the number piringan to the disk drive
- Single platter.
- Multiple platter.
• The Head mechanism
- Contact (floopy) was gotten by contact physically between head and the medium (the disk) for the operation read/wrote.
- Fixed Gap had the distance that continue to between head and the disk.
- Aerodynamic Gap (Winchester) had protective foil that aerodynamis between head and the disk so as the clearance head and the disk could be shortened.



Disk Access Time
•When a disk drive operates, the disk rotates with constant velocity.
• To be able to read and write heads must be at the beginning sector of the desired track.
• Selection includes tracks head movement on the movable head system or mechanism to
electronically at the head fixed head system.
- The time it takes to put head on the desired track is known as seek time.
• Once a track is chosen, the system will wait until the relevant sector rotates to fit the head.
- The time required by the sector to achieve the head is called rotational latency
Access time, the time it takes to be in a position ready to read or write.
• The number of seek time and rotational latency is the same as Access time.




RAID (Redundancy Array of Independent Disks)
• RAID (Redundancy Array of Independent Disk) was put forward to bring closer the chasm that was wide between the speed of the processor and elektromekanis the disk drive that was relatively slow.
• his strategy was by replacing the disk have a capacity big with several disks drive have a capacity small, and distributed the data in such a way as to enable data access from an amount drive simultaneously, that will increase the achievement of I/O and enabled the increase in the capacity easily.
• RAID overcame the problem standarisasi for the database plan with the numbering disk many.
• The RAID pattern consisted of six levels, zero as far as five.


Three general characteristics of RAID level, namely:
1. RAID is a set of disk drives that are considered by the operating system as a single logical drive.
2. The data is distributed to the physical drive array
3. Redundant disk capacity is used to store parity information, which ensures data recoverability in the event of failure disk.

RAID Level 0
• RAID level 0 is not RAID because family members do not use redundancy to improve performance.
• For RAID levels 0, user data and system data is distributed to all disks in the array.

RAID Level 1
• RAID level 1 RAID levels differ by 2 to 5 in the way of getting redundansinya.
• On the other RAID, some form of parity calculations are used to obtain redundancy.
• In RAID level 1, redundancy obtained by simply duplicate all the data.
• Some positive aspects of the organization RAID 1:
o Read requests can be served by one of the two disks containing the requested data, which has a seek time plus rotational latency to a minimum.
o Write requests require two strips related to the update, but this can be done in parallel.
o Recovery from failure is quite simple. If the drive fails, data can still be accessed from the second drive.

RAID level 2
• RAID levels 2 and 3 using the technique of parallel access.
• Dalam parallel access array,
• In parallel array access, all members participate in executing the disk every request I / O.
• player every drive is generally synchronized so that the disk head is always in the same position.

RAID level 3
• RAID organized RAID 3 in the same way with RAID 2, the difference is that RAID 3 requires only a single redundant disks, not depending on how big the array disknya.
• RAID 3 uses a parallel access to distributed data in the form of a small strip-Strik.
• Here Error-Correcting codes are not counted.


RAID level 4
• RAID 4 and 5 using the techniques of independent access.
• In the independent-access array, each disk members operate independently, so that request I / O can be fulfilled in parallel.
• high data transfer rate
• data striping Also used






Optical Memory
Produk the disk optis
1. CD (Compact Disk) is A disk that could not be removed that kept information audio that has
didigitasi. Sistem standar menggunakan disk 12 cm yang dapat merekam lebih dari 60 menit
waktu putar tanpa henti. The system uses a standard 12 cm disc that can record more than 60
minutes of continuous play time.
2. CD-ROM (Compact Disk Read Only Memory) is Disk that can not be removed to store
computer data. The system uses a standard 12 cm disc that can accommodate more than 550
Mbyte.
3. CD-I (Compact Disk Interactive) is a specification that is based on the use of CD-ROM.
This specification describes methods of providing audio, video, graphics, text, and executable
code on the machine CD-ROM.
4. DVI (Digital Video Interactive) is a representation of the technology to produce video
information is digitized and compressed. Representations can be stored on a CD or other disk
media. Current system using the CD and can store about 20 minutes of video on one disc.
5. WORM (Write One Read Many) is A disk that is easier than writing on CD-ROM, which
makes it commercially feasible to copy a CD. The popular measurement was 5.25 inchi that could
accommodate 200 as far as 800 Mbyte the data.
6 . Erasable Optical Disk = a disk that uses optical technology, but can be removed and easily
re-writable. There are two types of commonly used sizes: 3.25 inches and 5.25 inches.
Generally have a capacity of 650 Mbyte.

CD-ROM
• CD-ROM players have error-Correcting device to ensure that data is properly transferred from the disk into the computer.
• Disk is made of resin, such as polycarbonate, and coated with a highly reflective surface, usually aluminum.
• The information is recorded digitally published as a collection of microscopic pits on a reflective surface.
• The surface is protected from the dust disk and the friction with the clear coating.
Layout disk yang menggunakan constant angular velocity (CAV) Layout disk that uses constant angular velocity (CAV)
- Benefits CAV:blocks data can dialamati be directly by the track and sector. To move the
head to a specific address requires only a short head movement and short waiting times.
- Disadvantages CAV: the amount of data that can be stored on a remote track on the outside
similar to that can be stored with the track located near the center.
• Capacity CD-ROM storage is 774.57 Mbyte.
• Format CD-ROM block consists of the following fields.:
- Sync: sync field identifies the beginning of a block.
- Header: header consists of the address block and byte mode.
Mode 0 indicates a blank data field;
mode indicates the use of 1-Correcting error code and 2048 bytes of data;
2 mode indicates 2336 bytes of user data without error-Correcting codes.
• Data: the data is user data
• Auxiliary: additional user data in mode 2. In mode 1, these data additional user data in mode 2. In mode 1, this data is the error code-Correcting
• There is also a disk by using the layout constant linear velocity (CLV)
• Benefits CD-ROM:
- its information storage capacity far greater than the magnetic disk.
- It can be reproduced at low cost
- It can be removable. Most of the magnetic disk is not moveable.
• The lack of CD-ROM:
- CD-ROM can only read only (read only) and can not be updated.
- CD-ROM drive has access time is longer than the access time of magnetic disk drives.




• WORM
• WORM is a Write Once Read Many CD.
• Can be written once using a laser beam intensity was.
• technique used to prepare the disk is to use high-powered lasers.
• Using a constant angular velocity to provide faster access.
• Used for storage of archive documents and files in a large size.

• Can DeletedOptical Disk
• writable disk repeatedly
• Using technology magneto-optical system: in this system, laser energy is used in conjunction with the magnetic field to record and erase information.
• Using a constant angular velocity.
• The main advantage of this optical disk compared with magnetic disks:
- large capacity: a 5.25-inch optical disk can hold about 650 Mbyte of data.
- Portability: optical disk can be removed from the drive.
- Reliability
- Durable 

Magnetic tape
• Band system are uses reading and writing techniques similar to the system disk.
• Media system is flexible Mylar tape coated with magnetic oxide.
• Ribbon and tape drives is analogous to a tape recorder system.
• Medium-shaped ribbon of parallel tracks in small amounts.
• Sistem pita magnetik terbaru menggunakan 18 atau 36 track. latest magnetic tape systems using 18 or 36 tracks.
• Writeable and readable data in the form of continuous blocks called physical records on tape.
• Blocks separated bands separated by gaps known as inter-record gaps.