Computer Organization and Architecture: External Memory PDF

Computer Organization and Architecture Designing for Performance 11th Edition, Global Edition Chapter 7 External Memory Copyright © 2022 Pearson Education, Ltd. All Rights Res...

Computer Organization and Architecture Designing for Performance 11th Edition, Global Edition Chapter 7 External Memory Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Magnetic Disk A disk is a circular platter constructed of nonmagnetic material, called the substrate, coated with a magnetizable material – Traditionally the substrate has been an aluminium or aluminium alloy material – Recently glass substrates have been introduced Benefits of the glass substrate: – Improvement in the uniformity of the magnetic film surface to increase disk reliability – A significant reduction in overall surface defects to help reduce read- write errors – Ability to support lower fly heights – Better stiffness to reduce disk dynamics – Greater ability to withstand shock and damage Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Data are recorded on and later retrieved from the disk The write mechanism via a conducting coil named exploits the fact that the head electricity flowing through a In many systems there are two heads, a read head and a write head coil produces a magnetic During a read or write operation the field head is stationary while the platter rotates beneath it Magnetic Read The write head itself is made Electric pulses are sent to the write head and the of easily magnetizable and Write material and is in the shape resulting magnetic patterns of a rectangular doughnut Mechanisms are recorded on the surface with a gap along one side below, with different patterns and a few turns of for positive and negative conducting wire along the currents opposite side An electric current in the Reversing the direction of wire induces a magnetic field the current reverses the across the gap, which in turn direction of the magnetizes a small area of magnetization on the the recording medium recording medium Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Figure 7.1 Inductive Write/Magnetoresistive Read Head Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Figure 7.2 Disk Data Layout Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Figure 7.3 Comparison of Disk Layout Methods Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Figure 7.4 Legacy and Advanced Sector Formats Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Table 7.1 Physical Characteristics of Disk Systems Head Motion Platters Fixed head (one per track) Single platter Movable head (one per surface) Multiple platter Disk Portability Head Mechanism Nonremovable disk Contact (floppy) Removable disk Fixed gap Sides Aerodynamic gap (Winchester) Single sided Double sided Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Characteristics Fixed-head disk Removable disk – One read-write head per track – Can be removed and replaced – Heads are mounted on a fixed with another disk ridged arm that extends across all – Advantages: tracks ▪ Unlimited amounts of data are Movable-head disk available with a limited number – of disk systems One read-write head – ▪ A disk may be moved from one Head is mounted on an arm computer system to another – The arm can be extended or retracted – Floppy disks and ZIP cartridge disks are examples of removable disks Non-removable disk – Permanently mounted in the disk Double sided disk drive – Magnetizable coating is applied – The hard disk in a personal computer is a non-removable disk to both sides of the platter Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Disk Classification The head mechanism provides a classification of disks into three types The head must generate or Winchester Heads sense an electromagnetic field of sufficient magnitude to write Used in sealed drive assemblies and read properly that are almost free of contaminants The narrower the head, the Designed to operate closer to the closer it must be to the platter disk’s surface than conventional surface to function rigid disk heads, thus allowing – A narrower head means greater data density narrower tracks and therefore greater data Is actually an aerodynamic foil that density rests lightly on the platter’s surface The closer the head is to the when the disk is motionless disk the greater the risk of – The air pressure generated by a error from impurities or spinning disk is enough to make imperfections the foil rise above the surface Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Figure 7.5 Timing of a Disk I/O Transfer Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Disk Performance Parameters When the disk drive is operating the disk is rotating at constant speed To read or write the head must be positioned at the desired track and at the beginning of the desired sector on the track – Track selection involves moving the head in a movable-head system or electronically selecting one head on a fixed-head system – Once the track is selected, the disk controller waits until the appropriate sector rotates to line up with the head Seek time – On a movable–head system, the time it takes to position the head at the track Rotational delay (latency time) – The time it takes for the beginning of the sector to reach the head Bloc access time (access time) – The sum of the seek time, the latency time, and the transfer time Transfer time – Once the head is in position, the read or write operation is then performed as the sector moves under the head – This is the data transfer portion of the operation Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Table 7.2 Typical Hard Disk Drive Parameters HGST Ultrastar Characteristics HGST Ultrastar HE C15K600 Toshiba L200 Application Enterprise Data Center Laptop Capacity 12 TB 600 GB 500 GB Average seek time 8.0 ms read 2.9 ms read 11 ms 8.6 ms write 3.1 ms write Spindle speed 7200 rpm 15,030 rpm 5400 rpm Average latency 4.16 < 2 ms 5.6 ms Maximum sustained 255 MB/s 1.2 GB/s 3 GB/s transfer rate Bytes per sector 512/4096 512/4096 4096 Tracks per cylinder 8 6 4 (number of platter surfaces) Cache 256 MB 128 MB 16 MB Diameter 3.5 in (8.89 cm)s 2.5 in (6.35 cm) 2.5 in (6.35 cm) Maximum areal density 134 82 66 (Gb/cm2) Copyright © 2022 Pearson Education, Ltd. All Rights Reserved RAID Redundant Array of Independent Disks Consists of 7 levels Levels do not imply a hierarchical relationship but designate different design architectures that share three common characteristics: 1) Set of physical disk drives viewed by the operating system as a single logical drive 2) Data are distributed across the physical drives of an array in a scheme known as striping 3) Redundant disk capacity is used to store parity information, which guarantees data recoverability in case of a disk failure Copyright © 2022 Pearson Education, Ltd. All Rights Reserved  Table 7.3 RAID Levels Category Level Description Disks Data Availability Large I/O Data Small I/O Required Transfer Capacity Request Rate Striping 0 Nonredundant N Lower than single disk Very high Very high for both read and write Higher than RAID 2, Higher than single disk Up to twice that of a single Mirroring 1 Mirrored 2N 3, 4, or 5; lower than for read; similar to single disk for read; similar to RAID 6 disk for write single disk for write Parallel Access Much higher than 2 Redundant via N+m single disk; comparable Highest of all listed Approximately twice that Hamming code to RAID 3, 4, or 5 alternatives of a single disk Much higher than 3 Bit-interleaved N+1 single disk; comparable Highest of all listed Approximately twice that parity to RAID 2, 4, or 5 alternatives of a single disk Block-interleaved Much higher than Similar to RAID 0 for Similar to RAID 0 for read; 4 parity N+1 single disk; comparable read; significantly lower significantly lower than to RAID 2, 3, or 5 than single disk for write single disk for write Independent Block-interleaved Much higher than Similar to RAID 0 for Similar to RAID 0 for read; access 5 N+1 single disk; comparable read; lower than single generally lower than single distributed parity to RAID 2, 3, or 4 disk for write disk for write Block-interleaved Highest of all listed Similar to RAID 0 for Similar to RAID 0 for read; 6 dual distributed N+2 alternatives read; lower than RAID 5 significantly lower than parity for write RAID 5 for write N = number of data disks; m proportional to log N Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Figure 7.6 RAID Levels (1 of 2) Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Figure 7.6 RAID Levels (2 of 2) Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Figure 7.7 Data Mapping for a RAID Level 0 Array Copyright © 2022 Pearson Education, Ltd. All Rights Reserved RAID Addresses the issues of request patterns of the host system and layout of the data Level 0 Impact of redundancy does not interfere with analysis RAID 0 for High Data Transfer Capacity RAID 0 for High I/O Request Rate For applications to experience a high transfer rate two requirements For an individual I/O request for a must be met: small amount of data the I/O time is dominated by the seek time and 1. A high transfer capacity must rotational latency exist along the entire path between host memory and the A disk array can provide high I/O individual disk drives execution rates by balancing the I/O load across multiple disks 2. The application must make I/O requests that drive the disk array If the strip size is relatively large efficiently multiple waiting I/O requests can be handled in parallel, reducing the queuing time for each request Copyright © 2022 Pearson Education, Ltd. All Rights Reserved RAID Level 1 Characteristics Positive Aspects Differs from RAID levels 2 through 6 in A read request can be serviced by the way in which redundancy is either of the two disks that contains the achieved requested data Redundancy is achieved by the simple There is no “write penalty” expedient of duplicating all the data Recovery from a failure is simple, Data striping is used but each logical when a drive fails the data can be strip is mapped to two separate accessed from the second drive physical disks so that every disk in the array has a mirror disk that contains Provides real-time copy of all data the same data Can achieve high I/O request rates if the bulk of the requests are reads RAID 1 can also be implemented without data striping, although this is Principal disadvantage is the cost less common Copyright © 2022 Pearson Education, Ltd. All Rights Reserved RAID Level 2 Characteristics Performance Makes use of a parallel access An error-correcting code is calculated technique across corresponding bits on each data disk and the bits of the code are In a parallel access array all member stored in the corresponding bit disks participate in the execution of positions on multiple parity disks every I/O request Typically a Hamming code is used, Spindles of the individual drives are which is able to correct single-bit synchronized so that each disk head is errors and detect double-bit errors in the same position on each disk at The number of redundant disks is any given time proportional to the log of the number of Data striping is used data disks Would only be an effective choice in an – Strips are very small, often as environment in which many disk errors small as a single byte or word occur Copyright © 2022 Pearson Education, Ltd. All Rights Reserved RAID Level 3 Redundancy Performance Requires only a single redundant disk, In the event of a drive failure, the parity no matter how large the disk array drive is accessed and data is reconstructed from the remaining devices Employs parallel access, with data Once the failed drive is replaced, the distributed in small strips missing data can be restored on the new drive and operation resumed Instead of an error correcting code, a simple parity bit is computed for the In the event of a disk failure, all of the set of individual bits in the same data are still available in what is referred position on all of the data disks to as reduced mode Return to full operation requires that the Can achieve very high data transfer failed disk be replaced and the entire rates contents of the failed disk be regenerated on the new disk In a transaction-oriented environment performance suffers Copyright © 2022 Pearson Education, Ltd. All Rights Reserved RAID Level 4 Characteristics Performance Makes use of an independent access Involves a write penalty when an I/O technique write request of small size is performed − In an independent access array, Each time a write occurs the array each member disk operates management software must update not independently so that separate I/O only the user data but also the requests can be satisfied in parallel corresponding parity bits Thus each strip write involves two Data striping is used reads and two writes − Strips are relatively large To calculate the new parity the array management software must read the old user strip and the old parity strip Copyright © 2022 Pearson Education, Ltd. All Rights Reserved  RAID RAID Level 5 Level 6 Characteristics Characteristics Organized in a similar fashion to Two different parity calculations RAID 4 are carried out and stored in separate blocks on different disks Difference is distribution of the parity strips across all disks Advantage is that it provides extremely high data availability A typical allocation is a round- Three disks would have to fail robin scheme within the mean time to repair The distribution of parity strips (MTTR) interval to cause data to across all drives avoids the be lost potential I/O bottleneck found in Incurs a substantial write penalty RAID 4 because each write affects two parity blocks Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Table 7.4 RAID Comparison (1 of 2) Level Advantages Disadvantages Applications I/O performance is greatly improved The failure of just one Video production and by spreading the I/O load across many drive will result in all data editing channels and drives in an array being lost Image Editing 0 No parity calculation overhead is involved Pre- press applications Very simple design Any application requiring Easy to implement high bandwidth 100% redundancy of data means no Highest disk overhead Accounting rebuild is necessary in case of a disk of all RAID types Payroll failure, just a copy to the replacement disk (100%)—inefficient Financial Under certain circumstances, RAID 1 1 can sustain multiple simultaneous drive Any application requiring Failures very high availability Simplest RAID storage subsystem design Extremely high data transfer rates possible Very high ratio of ECC No commercial imple- The higher the data transfer rate disks to data disks mentations exist/not required, the better the ratio of data with smaller word commercially viable disks to ECC disks sizes— inefficient 2 Relatively simple controller design compared Entry level cost very high— to RAID levels 3, 4, & 5 requires very high transfer rate requirement to justify Very high read data transfer rate Transaction rate equal to Video production and live Very high write data transfer rate that of a single disk drive streaming at best (if spindles are Image editing (Table can be found Disk failure has an insignificant impact synchronized) on throughput Video editing on pages 230-231 in 3 Controller design is fairly Low ratio of ECC (parity) disks to data complex Prepress applications the textbook.) disks means high efficiency Any application requiring high throughput Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Table 7.4 RAID Comparison (2 of 2) Level Advantages Disadvantages Applications Very high Read data transaction rate Quite complex controller No commercial Low ratio of ECC (parity) disks to data design implementations disks means high efficiency Worst write transaction exist/not rate and Write aggregate commercially viable transfer rate 4 Difficult and inefficient data rebuild in the event of disk failure Highest Read data transaction rate Most complex controller File and application servers Low ratio of ECC (parity) disks to data design Database servers disks means high efficiency Difficult to rebuild in the Web, e- mail, and news Good aggregate transfer rate event of a disk failure (as compared to RAID servers 5 level 1) Intranet servers Most versatile RAID level Provides for an extremely high data More complex controller Perfect solution for mission fault tolerance and can sustain multiple design critical applications simultaneous drive failures Controller overhead to 6 compute parity addresses is extremely high (Table can be found on pages 230-231 in the textbook.) Copyright © 2022 Pearson Education, Ltd. All Rights Reserved SSD Compared to HDD SSDs have the following advantages over HDDs: – High-performance input/output operations per second (IOPS) – Durability – Longer lifespan – Lower power consumption – Quieter and cooler running capabilities – Lower access times and latency rates Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Table 7.5 Comparison of Solid State Drives and Disk Drives NAND Flash Drives Seagate Laptop Internal HDD File copy/write speed 200–550 Mbps 50–120 Mbps Power draw/battery life Less power draw, averages 2–3 watts, More power draw, averages 6–7 watts resulting in 30+ minute battery boost and therefore uses more battery Storage capacity Typically not larger than 1 TB for Typically around 500 GB and 2 TB Notebook size drives; 4 max for max for notebook size drives; 10 TB desktops max for desktops Cost Approx. $0.20 per GB for a 1-TB drive Approx. $0.03 per GB for a 4-TB drive Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Figure 7.8 Solid State Drive Architecture Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Practical Issues There are two practical issues peculiar to SSDs that are not faced by HDDs: Flash memory becomes SDD performance has a unusable after a certain number of writes tendency to slow down as the – Techniques for prolonging life: device is used  Front-ending the flash with a – The entire block must be read cache to delay and group from the flash memory and write operations placed in a RAM buffer  Using wear-leveling algorithms – Before the block can be written that evenly distribute writes back to flash memory, the entire across block of cells block of flash memory must be  Bad-block management erased techniques – The entire block from the buffer – Most flash devices estimate is now written back to the flash their own remaining lifetimes so memory systems can anticipate failure and take preemptive action Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Table 7.6 Optical Disk Products CD Compact Disk. A nonerasable disk that stores digitized audio information. The standard system uses 12-cm disks and can record more than 60 minutes of uninterrupted playing time. CD-ROM Compact Disk Read- Only Memory. A nonerasable disk used for storing computer data. The standard system uses 12-cm disks and can hold more than 650 Mbytes. CD-R CD Recordable. Similar to a CD-ROM. The user can write to the disk only once. CD-RW CD Rewritable. Similar to a CD-ROM. The user can erase and rewrite to the disk multiple times. DVD Digital Versatile Disk. A technology for producing digitized, compressed representation of video information, as well as large volumes of other digital data. Both 8 and 12 cm diameters are used, with a double-sided capacity of up to 17 Gbytes. The basic DVD is read-only (DVD-ROM). DVD-R DVD Recordable. Similar to a DVD-ROM. The user can write to the disk only once. Only one-sided disks can be used. DVD-RW DVD Rewritable. Similar to a DVD-ROM. The user can erase and rewrite to the disk multiple times. Only one- sided disks can be used. Blu-ray DVD High-definition video disk. Provides considerably greater data storage density than DVD, using a 405-nm (blue- violet) laser. A single layer on a single side can store 25 Gbytes. Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Compact Disk Read-Only Memory (CD-ROM) Audio CD and the CD-ROM share a similar technology – The main difference is that CD-ROM players are more rugged and have error correction devices to ensure that data are properly transferred Production: – The disk is formed from a resin such as polycarbonate – Digitally recorded information is imprinted as a series of microscopic pits on the surface of the polycarbonate ▪ This is done with a finely focused, high intensity laser to create a master disk – The master is used, in turn, to make a die to stamp out copies onto polycarbonate – The pitted surface is then coated with a highly reflective surface, usually aluminum or gold – This shiny surface is protected against dust and scratches by a top coat of clear acrylic – Finally a label can be silkscreened onto the acrylic Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Figure 7.9 CD Operation Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Figure 7.10 CD-ROM Block Format Copyright © 2022 Pearson Education, Ltd. All Rights Reserved CD-ROM CD-ROM is appropriate for the distribution of large amounts of data to a large number of users Because the expense of the initial writing process it is not appropriate for individualized applications The CD-ROM has two advantages: ▪The optical disk together with the information stored on it can be mass replicated inexpensively ▪The optical disk is removable, allowing the disk itself to be used for archival storage – The CD-ROM disadvantages: ▪It is read-only and cannot be updated ▪It has an access time much longer than that of a magnetic disk drive Copyright © 2022 Pearson Education, Ltd. All Rights Reserved CD Recordable CD Rewritable (CD-R) (CD-RW) Write-once read-many Can be repeatedly written and overwritten Accommodates applications in Phase change disk uses a material that which only one or a small number has two significantly different of copies of a set of data is needed reflectivities in two different phase states Disk is prepared in such a way Amorphous state that it can be subsequently written – Molecules exhibit a random once with a laser beam of modest- intensity orientation that reflects light poorly Crystalline state Medium includes a dye layer – Has a smooth surface that reflects which is used to change reflectivity and is activated by a light well high-intensity laser A beam of laser light can change the material from one phase to the other Provides a permanent record of Disadvantage is that the material large volumes of user data eventually and permanently loses its desirable properties Advantage is that it can be rewritten Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Figure 7.11 CD-ROM and DVD-ROM Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Figure 7.12 Optical Memory Characteristics Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Magnetic Tape Tape systems use the same reading and recording techniques as disk systems Medium is flexible polyester tape coated with magnetizable material Coating may consist of particles of pure metal in special binders or vapor-plated metal films Data on the tape are structured as a number of parallel tracks running lengthwise Serial recording – Data are laid out as a sequence of bits along each track Data are read and written in contiguous blocks called physical records Blocks on the tape are separated by gaps referred to as inter- record gaps Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Figure 7.13 Typical Magnetic Tape Features Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Table 7.7 LTO Tape Drives LTO-1 LTO-2 LTO-3 LTO-4 LTO-5 LTO-6 LTO-7 LTO-8 Release date 2000 2003 2005 2007 2010 2012 TBA TBA Compressed 200 GB 400 GB 800 GB 1600 GB 3.2 TB 8 TB 16 TB 32 TB capacity Compressed 40 80 160 240 280 400 788 1.18 transfer rate MB/s MB/s MB/s MB/s MB/s MB/s MB/s GB/s Linear density 4880 7398 9638 13,250 15,142 15,143 19,094 (bits/mm) Tape tracks 384 512 704 896 1280 2176 3,584 Tape length (m) 609 609 680 820 846 846 960 Tape width (cm) 1.27 1.27 1.27 1.27 1.27 1.27 1.27 Write elements 8 8 16 16 16 16 32 WORM? No No Yes Yes Yes Yes Yes Yes Encryption No No No Yes Yes Yes Yes Yes Capable? Partitioning? No No No No Yes Yes Yes Yes (Table can be found on page 241 in the textbook.) Copyright © 2022 Pearson Education, Ltd. All Rights Reserved Summary External Memory Chapter 7 Magnetic disk RAID – Magnetic read and write – RAID level 0 mechanisms – RAID level 1 – Data organization and – RAID level 2 formatting – RAID level 3 – Physical characteristics – RAID level 4 – Disk performance – RAID level 5 – RAID level 6 parameters Solid state drives Optical memory – Compact disk – SSD compared to HDD – Digital versatile disk – SSD organization – High-definition optical – Practical issues disks Magnetic tape Copyright © 2022 Pearson Education, Ltd. All Rights Reserved