Pertemuan 5

Jenis Memori External
 Magnetic Disk
 RAID
 Removable
 Optical
 CD-ROM
 CD-Writable (WORM)
 CD-R/W
 DVD
 Magnetic Tape
Magnetic Disk
 Metal atau plastic dilapisi dg material yg bersifat magnet (iron oxide)
 Jenis kemasan
 Floppy
 Winchester hard disk
 Removable hard disk
Format dan Organisasi Data
 Lingkaran konsentris atau track
 Ada Gap antar track
 Gap sempit, kapasitas bertambah
 Jumlah bit per track sama (kerapatan bervariasi)
 Kecepatan putar tetap
 Track dibagi menjadi beberapa sector
 Ukuran minimum block adalah satu sector
 Satu block bisa berisi lebih dari satu sector
Fixed/Movable Head Disk
 Fixed head
 Ada satu head (r/w) per track
 Head diletakkan pada tangkai yg tetap
 Movable head
 Hanya ada satu head per side
 Diletakkan pada tangkai yg dpt bergerak
Removable / Non removable
 Removable disk
 Dapat dilepas dari drive dan diganti dg disk lain
 Memberikan kapasitas simpanan yg tak terbatas
 Mudah melakukan transfer data antar sistem
 Nonremovable disk
 Terpasanang permanen dalam drive
Floppy Disk
 8”, 5.25”, 3.5”
 Kapasitas kecil
 sampai 1.44Mbyte (ada yg 2.88M)
 Lambat
 Umum dipakai
 Murah
Winchester Hard Disk (1)
 Dikembangkan oleh IBM di Winchester (USA)
 Dikemas dalam satu unit
 Berisi satu cakram atau lebih
 Head sangat kecil
 Handal
Winchester Hard Disk (2)
 Umum digunakan
 Murah
 Sbg external storage yg sangat cepat
 Kapasitas semakin besar
 Dalam orde GB
Removable Hard Disk
 ZIP
 Murah
 Banyak digunakan
 100MB
 JAZ
 Mahal
 1G
 L-120 (a: drive)
 Juga dpt untuk membaca 3.5” floppy
Pencarian Sector
 Harus dapat mengenali awal suatu track dan sector
 Format disk
 Menambahkan informasi tambahan
 Memberi tanda awal track dan sector
Karakteristik
 Fixed head atau movable head
 Removable disk atau fixed disk
 Single side atau double side
 Single platter atau multiple platter
 Mekanisme head
 Contact (Floppy)
 Fixed gap
 Flying (Winchester)
Multiple Platter
 Satu head per side
 Semua head di-join dan di-align
 Track-track yg setiap platter membentuk cylinder
 Data dipecah berdasarkan cylinder
 Mengurangi gerakan head
 Meningkatkan kecepatan (transfer rate)
Kecepatan
 Seek time
 gerakan head ke track yg dituju
 (Rotational) latency
 Putar platter sampai posisi data dibawah head
 Access time = Seek + Latency
 Transfer rate
RAID
 Redundant Array of Independent Disks
 Redundant Array of Inexpensive Disks
 Ada 6 level
 Tidak berhirarki
 Sejumlah disks (fisik) yg dipandang sbg satu drive (logical) oleh Sistem Operasil
 Data tersebar diantara disk fisik
RAID 0
 No redundancy
 Data striped across all disks
 Round Robin striping
 Increase speed
 Multiple data requests probably not on same disk
 Disks seek in parallel
 A set of data is likely to be striped across multiple disks
RAID 1
 Mirrored Disks
 Data is striped across disks
 2 copies of each stripe on separate disks
 Read from either
 Write to both
 Recovery is simple
 Swap faulty disk & re-mirror
 No down time
 Expensive
RAID 2
 Disks are synchronized
 Very small stripes
 Often single byte/word
 Error correction calculated across corresponding bits on disks
 Multiple parity disks store Hamming code error correction in corresponding position
 Lots of redundancy
 Expensive
 Not used
RAID 3
 Similar to RAID 2
 Only one redundant disk, no matter how large the array
 Simple parity bit for each set of corresponding bits
 Data on failed drive can be reconstructed from surviving data and parity info
 Very high transfer rates
RAID 4
 Each disk operates independently
 Good for high I/O request rate
 Large stripes
 Bit by bit parity calculated across stripes on each disk
 Parity stored on parity disk
RAID 5
 Like RAID 4
 Parity striped across all disks
 Round robin allocation for parity stripe
 Avoids RAID 4 bottleneck at parity disk
 Commonly used in network servers
 N.B. DOES NOT MEAN 5 DISKS!!!!!
Optical Storage CD-ROM
 Originally for audio
 650Mbytes giving over 70 minutes audio
 Polycarbonate coated with highly reflective coat, usually aluminum
 Data stored as pits
 Read by reflecting laser
 Constant packing density
 Constant linear velocity
CD-ROM Drive Speeds
 Audio is single speed
 Constant linier velocity
 1.2 ms-1
 Track (spiral) is 5.27km long
 Gives 4391 seconds = 73.2 minutes
 Other speeds are quoted as multiples
 e.g. 24x
 The quoted figure is the maximum the drive can achieve
Random Access on CD-ROM
 Difficult
 Move head to rough position
 Set correct speed
 Read address
 Adjust to required location
 (Yawn!)
CD-ROM for & against
 Large capacity (?)
 Easy to mass produce
 Removable
 Robust
 Expensive for small runs
 Slow
 Read only
Other Optical Storage
 CD-Writable
 WORM
 Now affordable
 Compatible with CD-ROM drives
 CD-RW
 Erasable
 Getting cheaper
 Mostly CD-ROM drive compatible
DVD - what’s in a name?
 Digital Video Disk
 Used to indicate a player for movies
 Only plays video disks
 Digital Versatile Disk
 Used to indicate a computer drive
 Will read computer disks and play video disks
 Dogs Veritable Dinner
 Officially - nothing!!!
DVD – technology
 Multi-layer
 Very high capacity (4.7G per layer)
 Full length movie on single disk
 Using MPEG compression
 Finally standardized (honest!)
 Movies carry regional coding
 Players only play correct region films
 Can be “fixed”
DVD – Writable
 Loads of trouble with standards
 First generation DVD drives may not read first generation DVD-W disks
 First generation DVD drives may not read CD-RW disks
 Wait for it to settle down before buying!
Foreground Reading
 Check out optical disk storage options
 Check out Mini Disk
Magnetic Tape
 Serial access
 Slow
 Very cheap
 Backup and archive
Digital Audio Tape (DAT)
 Uses rotating head (like video)
 High capacity on small tape
 4Gbyte uncompressed
 8Gbyte compressed
 Backup of PC/network servers