CMSC 411 - A. Sussman (from D. O'Leary) 1Computer Systems ArchitectureCMSC 411Unit 6 – Storage SystemsAlan SussmanNovember 23, 2004CMSC 411 - Alan Sussman 2Storage systems• We already know about four levels of storage:–registers– cache– memory–disk• but we've been a little vague on how these devices are interconnected• In this unit, we study– input/output units such as disks and tapes– buses to connect storage devices– I/O performance issues– design of file systems (won’t talk much about this)CMSC 411 - A. Sussman (from D. O'Leary) 2Disk and Tape TechnologiesCMSC 411 - Alan Sussman 4(Hard) Disks• What it is: – a collection of 1-20 platters (like 2-sided CD's) – between 1 and 8 inches in diameter – 2.5 & 3.5 inch most common today– rotating on a central spindle– with 500-2500 tracks on each surface– divided into (maybe) 64 sectors• older disks: all tracks have the same number of sectors• current disks: outer tracks have more sectors• larger diameter: best retrieval times• smaller diameter: cheaper and uses less powerCMSC 411 - A. Sussman (from D. O'Leary) 3CMSC 411 - Alan Sussman 5Disks (cont.) – Fig. 7.1•Used for– file storage– slowest level of virtual memory during program executionCMSC 411 - Alan Sussman 6Disks (cont.)• How information is retrieved:– Wait for previous requests to be filledTime = queuing delay– A movable arm is positioned at the correct cylinderTime = seek time– The system waits for the correct sector to appear under the armTime = rotational latency– Then a magnetic head senses• the sector number• the information recorded in the sector• an error correction codeCMSC 411 - A. Sussman (from D. O'Leary) 4CMSC 411 - Alan Sussman 7Disks (cont.)– and the information is transferred to a bufferTime = transfer time– The retrieval is handled by a disk controller, which may impose some extra overheadTime = controller time• Because all of this is so expensive, might also read the next sector or two, hoping that the next information needed is located there (prefetch or read ahead)CMSC 411 - Alan Sussman 8Example• Average disk access time (in millisec): average seek time + average rotational delay + transfer time + controller overhead1024 bytessector size.5 mscontroller overhead8000 RPMrotation speed10MB/sectransfer rate5 msaverage seek timeCMSC 411 - A. Sussman (from D. O'Leary) 5CMSC 411 - Alan Sussman 9Example (cont.)• average seek time = 5 ms• average rotational delay =• transfer time =• controller overhead = .5 ms• Total: 5 + 3.75 + .1 + .5 = 9.35 msmsRPSRPM75.3)60/000,8(5.0000,85.0==msbytesbytesMBKB1.sec10sec/1010sec/101473===−Computer Systems ArchitectureCMSC 411Unit 6 – Storage SystemsAlan SussmanNovember 30, 2004CMSC 411 - A. Sussman (from D. O'Leary) 6CMSC 411 - Alan Sussman 11Administrivia• HW #5 due today• Project due Friday– questions?• Quiz 3 scheduled for Dec. 7– practice quiz posted by tomorrow• Online course evaluation available at https://www.courses.umd.edu/online_evaluationCMSC 411 - Alan Sussman 12Last time• Speculation– can provide precise exceptions – ROB– can issue multiple instructions per clock, and commit multiple per clock– don’t speculate on expensive events (e.g. 2ndlevel cache misses)– can speculate through multiple branches• P6 microarchitecture– generate RISC-like micro-operations for each IA-32 instruction– out-of-order, speculative pipeline with ROBCMSC 411 - A. Sussman (from D. O'Leary) 7CMSC 411 - Alan Sussman 13Last time (cont.)• Storage systems– how a disk works• platters, tracks, cylinders, sectors– retrieval costs• queuing delay – wait for previous requests• seek time – find the right track• rotational latency – find the right sector• transfer time - read the data into a buffer (and sector number, ECC)• controller time – overhead in disk controllerCMSC 411 - Alan Sussman 14Technology gap between memory and disk – Fig. 7.5CMSC 411 - A. Sussman (from D. O'Leary) 8CMSC 411 - Alan Sussman 15Competitors to disks• solid state disks built from DRAMs (but needs constant power)• optical disks: CDs and DVDs• magnetic tapes: slower, but large capacity good for backups• automated tape libraries: juke box technology• flash memory – small, fast, low powerBusesCMSC 411 - A. Sussman (from D. O'Leary) 9CMSC 411 - Alan Sussman 17Buses• We've seen buses before, especially in the discussion of Tomasulo's algorithm• Main characteristic: Buses are shared by several data paths and therefore can be bottlenecks– CPU-memory buses: physically short, high speed, design optimized for performance– I/O buses: long, handle an unknown number of devices with unpredictable characteristicsCMSC 411 - Alan Sussman 18Typical bus transaction• When a READ is issued:– Bus begins in a wait state– Address sent on bus to memory, with control information to signal a read– When data is available, the wait signal is turned off and the data is transmitted• When a WRITE is issued:– Bus begins in a wait state– Address sent on bus to memory, with control information to signal a write– Then the data is transmitted, usually with no pauseCMSC 411 - A. Sussman (from D. O'Leary) 10CMSC 411 - Alan Sussman 19Bus options – Fig. 7.8asynchronoussynchronousClockingno – continuous connection cheaper and lower latencyyes – separate request and reply gets higher bandwidthSplit transactions?single (no arbitration)multiple (need arbitration)Bus masterssingle-word transfer is simplermultiple words have less overheadTransfer sizenarrower is cheaper (e.g., 8 bits)wider is faster (e.g., 64 bits)Data widthmultiplex address and data linesseparate address and data linesBus widthLow costHigh performanceOptionCMSC 411 - Alan Sussman 20Who issues READs and WRITEs?•The bus master does• If the bus is between CPU and memory, then the CPU is the bus master• If it is an I/O bus, then there might be several devices, so several bus masters, and they compete for time slices on the bus– In this case, buses are often packet switched - each device divides its message into fixed length packets, and takes turns with other devices that are transmittingCMSC 411 - A. Sussman (from D. O'Leary) 11CMSC 411 - Alan Sussman 21Synchronous vs. asynchronous buses• Buses that are clocked (synchronous) send data and addresses at fixed times, so sender and receiver always know what to expect – Makes them fast and cheap– But