高等計(jì)算機(jī)系統(tǒng)結(jié)構(gòu)-展示頁(yè)
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【正文】 ow improvement in seek, rotation (8%/yr) ? Time to read whole disk Year Sequentially Randomly (1 sector/seek) 1990 4 minutes 6 hours 2022 12 minutes 1 week(!) 2022 56 minutes 3 weeks (SCSI) 2022 171 minutes 7 weeks (SATA) 北京大學(xué)計(jì)算機(jī)科學(xué)技術(shù)系 計(jì)算機(jī)系統(tǒng)結(jié)構(gòu)教研室 存貯設(shè)備 : 磁盤(pán) 扇區(qū)（ Sector） 磁道（ Track） 柱面（ Cylinder） 磁頭（ Head） 盤(pán)片（ Platter） ? 用途 ? 長(zhǎng)期、非易失的存貯 ? 在存貯層次中，大容量、廉價(jià)、較慢的級(jí)別 ? 特性 ? 尋道時(shí)間 (平均 8 ms左右 ) ? 定位延遲（ positional latency） ? 旋轉(zhuǎn)延遲（ rotational latency） ? 傳輸率 ? 大約每毫秒一個(gè)扇區(qū) (515 MB/s) ? 成塊 ? 容量 ? Gigabytes ? 三年四倍 7200 RPM = 120 RPS = 8 ms per rev ave rot. latency = 4 ms 128 sectors per track = ms per sector 1 KB per sector = 16 MB / s 響應(yīng)時(shí)間 = 排隊(duì) + 控制器 + 尋道 + 旋轉(zhuǎn) + 傳輸 服務(wù)時(shí)間 北京大學(xué)計(jì)算機(jī)科學(xué)技術(shù)系 計(jì)算機(jī)系統(tǒng)結(jié)構(gòu)教研室 磁盤(pán)面密度： Disk Areal Density ? Bits recorded along a track ? Metric is Bits Per Inch (BPI) ? Number of tracks per surface ? Metric is Tracks Per Inch (TPI) ? Disk designs brag about bit density per unit area ? Metric is Bits Per Square Inch: Areal Density = BPI x TPI Ye a r A re a l D e n s i t y1973 2 1979 8 1989 63 1997 3 , 0 9 0 2022 1 7 , 1 0 0 2022 1 3 0 , 0 0 0 1101001 , 0 0 01 0 , 0 0 01 0 0 , 0 0 01 , 0 0 0 , 0 0 01970 1980 1990 2022 2022Y e a rAreal Density北京大學(xué)計(jì)算機(jī)科學(xué)技術(shù)系 計(jì)算機(jī)系統(tǒng)結(jié)構(gòu)教研室 1 inch disk drive! ? 2022 IBM MicroDrive: ? ” x ” x ” ? 1 GB, 3600 RPM, 5 MB/s, 15 ms seek ? Digital camera, PalmPC? ? 2022 MicroDrive? ? 9 GB, 50 MB/s! ? Assuming it finds a niche in a successful product ? Assuming past trends continue 北京大學(xué)計(jì)算機(jī)科學(xué)技術(shù)系 計(jì)算機(jī)系統(tǒng)結(jié)構(gòu)教研室 磁帶與磁盤(pán) ?縱向磁帶使用與磁盤(pán)相同的工藝技術(shù)；同樣的密度改進(jìn) ? 磁盤(pán)的磁頭在磁盤(pán)表面上方，而磁帶在表面上（接觸） ? 磁盤(pán)固定、磁帶可移動(dòng) ?內(nèi)在的成本 性能 依賴(lài)于 其幾何形狀： ? 具有一定縫隙固定的旋轉(zhuǎn)盤(pán)片 (隨機(jī)訪(fǎng)問(wèn)、限制尺寸、 1 media / reader) 與 ? 可移動(dòng)的長(zhǎng)帶、纏繞在線(xiàn)軸上 (順序訪(fǎng)問(wèn)、“無(wú)限”長(zhǎng)度、 multiple / reader) ?新工藝技術(shù)趨勢(shì) ? 螺旋掃描 (錄像帶 , 數(shù)字錄音帶 ) ? 與磁帶成一定角度旋轉(zhuǎn)磁頭來(lái)改進(jìn)密度 北京大學(xué)計(jì)算機(jī)科學(xué)技術(shù)系 計(jì)算機(jī)系統(tǒng)結(jié)構(gòu)教研室 當(dāng)前磁帶的缺陷 ? 磁帶磨損 ? 螺旋式磁帶上百次 縱向磁帶上千次 ? 磁頭磨損 ? 螺旋式 2022小時(shí) ? 兩者都必須考慮 經(jīng)濟(jì) /可靠 ? 纏繞、退帶、裝帶、旋轉(zhuǎn)次數(shù)多 并不是必然如此，只是目前為止尚沒(méi)有進(jìn)一步市場(chǎng)需求 ? 針對(duì)歸檔需求設(shè)計(jì) 北京大學(xué)計(jì)算機(jī)科學(xué)技術(shù)系 計(jì)算機(jī)系統(tǒng)結(jié)構(gòu)教研室 Current Drawbacks to Tape ? Tape wear out: ? Helical 100s of passes to 1000s for longitudinal ? Head wear out: ? 2022 hours for helical ? Both must be accounted for in economic / reliability model ? Bits stretch ? Readers must be patible with multiple generations of media ? Long rewind, eject, load, spinup times。 in 1992, ASCII of all books = 30 TB ? Exchange up to 450 tapes per hour (8 secs/tape) ? to Mbyte/sec per reader, up to 10 readers feet feet 8200 pounds, kilowatts 北京大學(xué)計(jì)算機(jī)科學(xué)技術(shù)系 計(jì)算機(jī)系統(tǒng)結(jié)構(gòu)教研室 Flash: The future of disks? ? Advent of digital cameras, mp3 players, has driven market for lowcost nonvolatile flash memory ? Other promising technologies in development: phasechange RAM, magic RAM (core returns!), but flash has big lead ? In 2022, several announcements of flashbased disk replacement for laptops/servers ? SanDisk, Samsung, I/O fusion, … ? Flash drive advantages: ? Lower power (no moving parts) ? Much faster seek time, 100X IOs per second (no moving parts) ? Greater reliability (no moving parts) ? Lower noise (no moving parts) ? Flash disadvantages ? Cost (20100x disk cost/GB) ? Slow writes with current design (petitive with disks) ? write endurance not an issue for most applications since use writeleveling to spread wear around blocks on chip ? Potential benefit of flash hidden behind standard disk interface ? Scope for massive rethinking of storage architecture if nonvolatile moves into memory hierarchy and accessed via processor loads/stores not seek/read/write 北京大學(xué)計(jì)算機(jī)科學(xué)技術(shù)系 計(jì)算機(jī)系統(tǒng)結(jié)構(gòu)教研室 名詞解釋?zhuān)?Dependability ? Dependability – The quality of delivered service such that reliance can justifiably be placed on this service. ? Failure ? Error ? Fault ? Service ? Specified service ? Delivered service ? Service acplishment ? Service interruption 北京大學(xué)計(jì)算機(jī)科學(xué)技術(shù)系 計(jì)算機(jī)系統(tǒng)結(jié)構(gòu)教研室 名詞解釋 ? Reliability – A measure of the continuous service acplishment (or, equivalently, of the time to failure) from a reference initial instant. ? MTTF – mean time to failure ? MTTR mean time to repaire ? Availability – A measure of the service acplishment with respect to the alternation between the two state of acplishment and interruption. ? MTBF – mean time between failures ? Availability = MTTF/MTBF 北京大學(xué)計(jì)算機(jī)科學(xué)技術(shù)系 計(jì)算機(jī)系統(tǒng)結(jié)構(gòu)教研室 小尺寸磁盤(pán)的優(yōu)勢(shì) 低 成本 /MB 高 MB/大小 高 MB/瓦特 低 成本 /驅(qū)動(dòng)器 成本和環(huán)境情況 北京大學(xué)計(jì)算機(jī)科學(xué)技術(shù)系 計(jì)算機(jī)系統(tǒng)結(jié)構(gòu)教研室
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