Chapter 5Secondary Storages 國立聯合大學 電子工程學系 蕭裕弘
Chapter Goals • 說明電腦系統常用的儲存設備的特性 • 介紹兩種常見的磁碟系統與其作業方式 • 介紹數種不同的光碟系統與其作業方式 • 介紹 flash memory 與相關產品 • 介紹磁帶儲存系統與其作業方式
1. Introduction • Different types and different purposes of computer storages: • Primary access from the CPU or secondary (indirect) access by the CPU, which was based primarily on speed of access to the memory. • Volatile or non-volatile, which is based on the technology (magnetic vs. electrical, etc.) • Read-only memory, WORM, or read-write, again based on technology. • Random-Access or Sequential-Access, which is based on the mechanism of reading the memory. • Block or File access. • Media categories, e.g., semiconductor storage, optical storage, magneto-optical storage, and magnetic storage, which is mainly disk-based, rather than memory-based.
Primary vs. Secondary Storage • In traditional parlance, primary storage contains data that are actively being used (for example, the programs currently being run and the data they are operating on). • It is typically high-speed, relatively small. • It is often (but not always) volatile. • It is sometimes referred to as "Main Memory." • Secondary storage, also known as peripheral storage or auxiliary storage, is where the computer stores information that is not necessarily in current use. • It is typically slower and higher-capacity than primary storage. • It is almost always non-volatile.
Random vs. Sequential Access • Random-access media has the property of accessing any portion at any time. Semiconductor memory and magnetic disk are examples of this type of storage. • Sequential-access media by contrast must be read in sequence regardless of the desired content. Magnetic tape and certain types of flash memory have this property.
Block vs. File Access • In disk storage, these are the two primary access methods: • Block access means that the disk is divided into normally equal-sized blocks which are accessed at random by the operating system. Physical representation • File access contains an abstraction of files and directories which can be used to refer to storage content. Logical representation
2. Magnetic Disk Systems • Magnetic disks are the most widely used storage media on today’s computers. • Writing Heads • Reading Heads • Storage Media (e.g., computer disks)
Floppy Disks • 8” – 800 KB (max.) • 5.25” – 1.2 MB (max.) • 3.5” – 1.44 MB (max.)
Tracks, Sectors, and Clusters • Tracks • Narrow rings which the read/write head encodes with 0s and 1s when it writes data and programs to the disk. Each track has a number. • In order to work on a particular computer system, floppy disks must be formatted. • Formatting divides the disk surface into pie-shaped sectors, typical 512 bytes per sector. • On many PC systems, the part of a track that crosses a fixed number of contiguous sectors (typically, 2 ~ 8) forms a unit called cluster or disk block. • A cluster is the smallest addressable area on a disk.
Cylinders • On a hard disk, a cylinder is made of all the tracks of the same number from all the metal disks that make up the "hard disk".
Disk Performance - 1 • There are three primary factors that determine hard drive performance: • Seek time • A measure of the speed with which the drive can position its read/write heads over any particular data track. • The standard way to measure seek time is to time a large number of disk accesses to random locations, subtract the latency (see below) and take the mean. • Rotational latency • The time that elapses between the moment when the read/write head settles over the desired data track and the moment when the first byte of the required data appears under the head. • On average, latency is always equal to one half of the rotational period. • Internal data transfer rate • The speed with which the drive's internal read channel can transfer data from the magnetic media.
Disk Performance - 2 • Subsidiary performance factors include: • Access time • The sum of the seek time and the latency. • The external data rate • The speed with which the drive can transfer data from its buffer to the host computer system. • Command overhead • The time it takes the drive electronics to interpret instructions from the host computer and issue commands to the read/write mechanism. In modern drives it is negligible. In some text books, disk access time is the sum of the seek time, the rotational latency, and the data movement time (the time required for the system to read the data from the disk and transfer it to the RAM, or to transfer the data to be written to the disk from RAM and then store it on the disk).
Disk Standards • PC hard disk systems can use several different interface standards which determine performance characteristics: • The density with which data can be packed onto the disk. • The speed of disk access. • How large the disk can be. • The way the disk drive interface with other hardware. • Three standards dominate the market: • Advanced Technology Attachment/Integrated Drive Electronics (ATA/IDE) • Small computer system interface (SCSI) • SATA
Disk Standards – ATA/IDE • Common versions of ATA/IDE:
Disk Standards – SCSI • The mainstream implementations of SCSI (in chronological order) are as follows, using common parlance: • SCSI-1: 5 MB/Sec max. • SCSI-2: 20 MB/Sec max. • SCSI-3: 40 MB/Sec max. • Ultra-2 SCSI: 80 MB/Sec max. • Ultra-3 SCSI: 160 MB/Sec max. • Ultra-320 SCSI : 320 MB/Sec max. • Ultra-640 SCSI : 640 MB/Sec max.
SCSI vs. EIDE • SCSI offers more performance than EIDE but at a price. • Termination is more complex but expansion not too difficult. • Having more than 4 (or in some cases 2) IDE drives can be complicated, with wide SCSI you can have up to 15 per adapter. Some SCSI host adapters have several channels thereby multiplying the number of possible drives even further. • For SCSI you have to dedicate one IRQ per host adapter which can control up to 15 drives. With EIDE you need one IRQ for each channel (which can connect up to 2 disks, master and slave) which can cause conflict. SCSI EIDE
Disk Standards – SATA • SATA has at least three main advantages over its predecessor, namely speed, cable management and hot-swappability. • Initially Serial ATA was released at 150 megabytes per second but it is designed to scale up quite substantially from there. • Serial ATA II will double throughput to 300 MB/s and then 600 MB/s is planned for around 2007.
Disk Partitioning • Disk partitioning is the creation of logical divisions upon a hard disk drive that allows one to apply operating system-specific logical formatting.
RAID • A Redundant Array of Independent Disks (RAID) is a system of using multiple hard drives for sharing or replicating data among the drives. • The benefit of RAID is increased data integrity, fault-tolerance and/or performance, over using drives singularly. Disk mirroring Disk striping
3. Optical Discs • An optical disc is a flat, circular, plastic disc coated with material on which bits may be stored in the form of highly reflective areas and significantly less reflective areas, from which the stored data may be read when illuminated with a narrow-beam source, such as a laser diode. • Compact disc • DVD
Compact Discs • A compact disc (or CD) is an optical disc used for storing digital data. It was originally invented for digital audio and is also used as a data storage device, a CD-ROM. • The compact disc was developed in 1979 by Philips and Sony. • A 120mm disc can store about 74 minutes of music or about 650 megabytes of data. • Discs that can store about 700 megabytes (80 minutes of music) have become more common however. • The mini-CD is 80mm in diameter, holds about 140MB of data or 21 minutes of audio, and has the exact same data format as the larger one.
CD-ROM 的規格 • 資料傳輸速度 • 1X: 150 Kbytes/Sec • 52X: 52 * 150 Kbytes/Sec = 7,800 Kbytes/Sec • 面板播放 • 可直接由光碟機面板播放音樂光碟 • 進片方式 • 吸片式、拖盤式 (輔助卡榫) • 連接介面 • IDE/ATAPI 、SCSI
Recordability of CD • Compact discs cannot be easily recorded, as they are manufactured by etching a glass plate and using that plate to press metal. • There are also CD-recordable discs, which can be recorded by a laser beam using a CD-R writer, and can be played on most compact disc players. • CD-R recordings are permanent and cannot be recorded more than once, so the process is also called "burning" a CD. • CD-RW is a medium that allows multiple recordings on the same disc over and over again. • A CD-RW does not have as great a difference in the reflectivity of lands and bumps as a pressed CD or a CD-R, so many CD audio players cannot read CD-RW discs. W Speed*R/W Speed*R Speed
DVD • DVD is an optical disc storage media format that is used for playback of movies with high video and sound quality and for storing data. • A DVD can contain: • DVD-Video (containing movies (video and sound)) • DVD-Audio (containing high-definition sound) • DVD-Data (containing data) • The disc medium can be: • DVD-ROM (read only, manufactured by a press) • DVD+R/RW (R=Recordable once, RW = ReWritable) • DVD-R/RW • DVD-RAM (random access rewritable)
DVD Discs • The disc may have one or two sides, and one or two layers of data per side; the number of sides and layers determines the disc capacity: • DVD-5: single sided, single layer, 4.7 gigabytes (GB), or 4.38 gibibytes (GiB) • DVD-9: single sided, double layer, 8.5 GB (7.95 GiB) • DVD-10: double sided, single layer on both sides, 9.4 GB (8.75 GiB) • DVD-14: double sided, double layer on one side, single layer on other, 13.2 GB (12.3 GiB) • DVD-18: double sided, double layer on both sides, 17.1 GB (15.9 GiB)
Magneto-Optical (M-O) Discs • Magneto-Optical disc is an optical disc format that uses a combination of optical and magnetic technologies. • The sizes of discs are usually 3.5 inch or 5.25 inch, and disk capacities are usually one of 128MB/230MB/540MB/640MB/1.3GB/2.6GB. • A special Magneto-optical drive is required to read these discs. • During writing, the magneto-optical disc needs to be erased, written, and then verified, which is a time-consuming process. Eventually, a technology called direct overwrite improved perfomance by eliminating the erase step.
4. Flash Memory Cards • Flash memory is a form of EEPROM that allows multiple memory locations to be erased or written in one programming operation. • Normal EEPROM only allows one location at a time to be erased or written, meaning that flash can operate at higher effective speeds when the systems using it read and write to different locations at the same time. • All types of flash memory and EEPROM wear out after a certain number of erase operations. • Types of flash memory cards: • Memory Stick (MS) • CompactFlash (CF) • SmartMedia (SM) • Secure Digital (SD) • Multi Media Card (MMC) • xD-Picture (xD)
Flash Memory Drivers • Flash memory drivers are referred to as solid-state drives, because they have no moving parts. • More resistant to shock and vibration than conventional drivers. • They also have a longer expected life than magnetic drivers.
5. Magnetic Tape Systems • Magnetic tape is an information storage medium consisting of a magnetizable coating on a thin plastic strip. • Magnetic tape was first used to record data in 1951 on the UNIVAC I. • Tape remains a viable alternative to disk due to its higher bit density and lower cost per bit. Tape has historically offered enough advantage in these two areas above disk storage to make it a viable product. • Because of tape’s sequential-access property, it is used primarily today to back up the contents of other storage systems.