2021_CO1201_Lecture_06_External_Memory_801f484db66285a4ff2d8cdee3bc17a0.pdf

CO1201 Introduction to Computer Systems Department of Computer Engineering Faculty of Engineering University of Sri Jayewardenepura By Dr. Randima Dinalankara EXTERNAL MEMORY 2 External memory devices Magnetic tape Magnetic disk Redund...

CO1201 Introduction to Computer Systems Department of Computer Engineering Faculty of Engineering University of Sri Jayewardenepura By Dr. Randima Dinalankara EXTERNAL MEMORY 2 External memory devices Magnetic tape Magnetic disk Redundant Array of Independent Disks(RAID) Optical Memory Solid State Drives (SSD) 3 Computer Engineering Department Magnetic Tap Use magnetizable material A flexible tape Data stored in parallel tracks – Nine tracks – One byte per track 4 Computer Engineering Department Magnetic Tap (cont.) Advantage Low cost Disadvantages Has to be access sequentially Needed a manual positioning 5 Computer Engineering Department Magnetic Disk A circular platter Coated with magnetizable material Use conducting coil (head) to read/write Single head for both read and write – Floppy disk Dual heads for read and write 6 Computer Engineering Department Magnetic Disk (cont.) Data stored to – Concentric rings → tracks – Adjacent rings separated by gaps – Track separate into sectors 512 bytes for a universal sector – Based on sector size, constant angular velocity multiple zone velocity 7 Computer Engineering Department 8 9 Magnetic Disk (cont.) Data in a sector – Special data pattern Beginning and End – Disk formatting From the reference text book Winchester disk formatting 17 Bytes - Sector starts 7 Bytes - Sector ID 41 Bytes - Gap 515 Bytes - Data (512 bytes true data) 20 Bytes - Sector ends 10 Computer Engineering Department 11 Magnetic Disk (cont.) Physical characteristic Head movability fixed-head movable-head Mounting non removable removable method No. of sides Single sided Double sided No. platters Single platter Multiple platters 12 Computer Engineering Department Magnetic Disk (cont.) Head mechanism 1. Fixed distance to disk 2. Physically contacting 3. Air gap change during the operation time Winchester disk 13 Computer Engineering Department Hard disk reading mechanism Constant Angular Velocity Multiple Zone Recording (CAV) (MZR) 14 Hard disk reading mechanism Constant Angular Velocity Multiple Zone Recording (CAV) (MZR) Different Zones Data density Multiple tracks belong to zone High - inner sectors Data density is constant Low - outer sectors Advantage Advantage More storage capacity Simplicity Disadvantage Disadvantage Complext circuitry is needed Less data storage 15 RAID Redundant Array of Independent Disks Features 1. Multiple physical disk drivers Computer view as single logical driver 2. Data distributed between the disks 3. Parity information stored in an extra disk 16 Computer Engineering Department RAID (cont.) Seven different RAID levels. Going to overview RAID level 0, 1, 2, and 3 in this lecture. RAID level 0 – Not a true RAID system. – Data distributed between disks – I/O transfer rate increased. 17 Computer Engineering Department RAID 0 18 RAID (cont.) RAID level 1 – Duplicate all data. – Total no. disk = 2 × no. of data disks – Improve the data seek time – Data recovery is simple when a disk fails 19 Computer Engineering Department RAID 1 20 RAID (cont.) RAID level 2 – Total no. disks = no. of data disks + additional disk(s) – Additional disk for parity data Correct single bit error – Strip size is very small (single byte) Use all disk simultaneously 21 Computer Engineering Department RAID 2 22 RAID (cont.) RAID level 3 – Total no. disks = no. data disks + 1 – Use very small strips – High data transfer rate – All disk use in parallelly – Only one I/O request possible – Data recovery is fairly easy 23 Computer Engineering Department RAID 3 24 RAID 4 What is the difference between the RAID 3 and 4? 25 RAID 5 No dedicated parity disk. Distribute the parity data in all the hard disk. 26 Optical Memory Compact Disk (CD) Digital Versatile Disk (DVD) Blu-ray DVD 27 Computer Engineering Department Optical Memory - CD ROM Developed in 1983 12 cm diameter disk More than 60 minutes of audio /680 MB Spiral track Data representation – Check surface in regular interval – If pit-land/ land-pit occur, then 1 – Else 0 28 Computer Engineering Department Optical Memory - CD ROM (cont.) Advantages – Removable – Inexpensive media for replication Disadvantages – Read-only – Longer access time 29 Computer Engineering Department Optical Memory - CD-R & CD-RW CD recordable (CD-R) – User can write data once – A laser changes a dye layer – Use as a backup media CD Rewritable (CD-RW) – Phase change technique – Writing capabilities eventually loses 30 Computer Engineering Department Optical Memory - DVD Greater capacity than CD-ROMs – Reduced Bit distance (4.7 GB) – Dual layer (8.5 GB) – Two sides (17 GB) Random access capability Available in – Writable – Read-only 31 Computer Engineering Department Optical Memory - Blu-Ray Use ultra-violet wavelength Data layer is closer to lens Higher bit density 25 GB on single layer Three versions – Read only (BR-ROM) – Recordable (BR-R) – Rerecordable (BR-RE) 32 Computer Engineering Department SSD Solid State Device (SSD) Developed based on flash memory – Improved in bit density – Cost has dropped Built with semiconductors – Transistor Strong competitor with HDD 33 Computer Engineering Department SSD (cont.) Advantages over HDD 1. Higher I/O operations rate 2. Longer Durability 3. Lower power consumption 4. Quite and cooler 5. Lower access time and latency 34 Computer Engineering Department SSD (cont.) Practical issues 1. Tendency to slow down When more data is been written 2. Unusable after writing data for certain no. of times 35 Computer Engineering Department Self study topics Track gap differences and wave lengths used in CD, DVD, and Blu-ray disks (Optical memory - Optical Memory Characteristics) NOR flash memory vs NAND flash memory (Solid State Device -Flash memory section) Techniques used to extend SSD life (Solid State Device -Practical Issues section) 36 Computer Engineering Department In summary, External memory – Hard disk track, sectors, etc. – RAID systems – SSD – Optical drives 37 Computer Engineering Department Next week Input Output Module 38 Computer Engineering Department