G22.2233 L13 Disks, RAIDs, and I/O Systems. 1 Banikazemi, NYU, 2007CS G22.2233 Computer Systems Design Spring 2007Lecture 13: Disks, RAIDs, and I/O SystemsMohammad Banikazemi[Slides from Prof. Mary Jane Irwin, PSU Adapted fromComputer Organization and Design, Patterson & Hennessy, © 2005, UCB]G22.2233 L13 Disks, RAIDs, and I/O Systems. 2 Banikazemi, NYU, 2007Review: Major Components of a ComputerProcessorControlDatapathMemoryDevicesInputOutputCacheMain MemorySecondary Memory(Disk)G22.2233 L13 Disks, RAIDs, and I/O Systems. 3 Banikazemi, NYU, 2007Magnetic Disk PurposeO Long term, nonvolatile storageO Lowest level in the memory hierarchy- slow, large, inexpensiveGeneral structureO A rotating platter coated with a magnetic surfaceO A moveable read/write head to access the information on the disk Typical numbersO 1 to 4 (1 or 2 surface) platters per disk of 1” to 5.25” in diameter (3.5” dominate in 2004)O Rotational speeds of 5,400 to 15,000 RPMO 10,000 to 50,000 tracks per surface- cylinder - all the tracks under the head at a given point on all surfacesO 100 to 500 sectors per track- the smallest unit that can be read/written (typically 512B)TrackSectorG22.2233 L13 Disks, RAIDs, and I/O Systems. 4 Banikazemi, NYU, 2007Magnetic Disk Characteristic Disk read/write components1. Seek time: position the head over the proper track (3 to 14 ms avg)- due to locality of disk references the actual average seek time may be only 25% to 33% of the advertised number2. Rotational latency: wait for the desired sector to rotate under the head (½ of 1/RPM converted to ms)- 0.5/5400RPM = 5.6ms to 0.5/15000RPM = 2.0ms3. Transfer time: transfer a block of bits (one or more sectors) under the head to the disk controller’s cache (30 to 80 MB/s aretypical disk transfer rates)- the disk controller’s “cache” takes advantage of spatial locality in disk accesses– cache transfer rates are much faster (e.g., 320 MB/s)4. Controller time: the overhead the disk controller imposes in performing a disk I/O access (typically < .2 ms)SectorTrackCylinderHeadPlatterController+CacheG22.2233 L13 Disks, RAIDs, and I/O Systems. 5 Banikazemi, NYU, 2007Typical Disk Access Time  The average time to read or write a 512B sector for a disk rotating at 10,000RPM with average seek time of 6ms, a 50MB/sec transfer rate, and a 0.2ms controller overheadIf the measured average seek time is 25% of the advertised average seek time, then The rotational latency is usually the largest component of the access timeG22.2233 L13 Disks, RAIDs, and I/O Systems. 6 Banikazemi, NYU, 2007Typical Disk Access Time If the measured average seek time is 25% of the advertised average seek time, thenAvg disk read/write = 6.0ms + 0.5/(10000RPM/(60sec/minute) )+ 0.5KB/(50MB/sec) + 0.2ms = 6.0 + 3.0 + 0.01 + 0.2 = 9.21msAvg disk read/write = 1.5 + 3.0 + 0.01 + 0.2 = 4.71ms The average time to read or write a 512B sector for a disk rotating at 10,000RPM with average seek time of 6ms, a 50MB/sec transfer rate, and a 0.2ms controller overhead The rotational latency is usually the largest component of the access timeG22.2233 L13 Disks, RAIDs, and I/O Systems. 7 Banikazemi, NYU, 2007Magnetic Disk Examples (www.seagate.com)1.41612/8/191”x4”x5.8”600,0008.5r-9.5w1.0r-1.2w32-587,20042003.5Seagate ST321.5r-2.0w0.2r-0.4wMinimum seek (ms)3457-86Transfer rate (MB/sec)0.4”x2.7”x3.9”1”x4”x5.8”Dimensions (inches)330,0001,200,000MTTF (hours@25oC)0.21.9Weight (pounds)174GB/watt2.4/1/0.420?/12/-Power: op/idle/sb (watts)103GB/cu.inch12r-14w3.6r-3.9wAverage seek (ms)5,40015,000Rotation speed (RPM)28# of surfaces (heads)4073.4Capacity (GB)2.53.5Disk diameter (inches)Seagate ST94Seagate ST37CharacteristicG22.2233 L13 Disks, RAIDs, and I/O Systems. 8 Banikazemi, NYU, 2007Disk Latency & Bandwidth Milestones Disk latency is one average seek time plus the rotational latency. Disk bandwidth is the peak transfer time of formatted data from the media (not from the cache).5.78.812.717.148.3Latency (msec)8624940.6Bandwidth (MB/s)SCSISCSISCSISCSIST-412Interface2.53.03.55.255.25Diameter (inches)73.49.14.31.40.03Capacity (Gbytes)20031998199419901983Year1500010000720054003600RSpeed (RPM)SG ST37SG ST39SG ST15SG ST41CDC WrenPatterson, CACM Vol 47, #10, 2004G22.2233 L13 Disks, RAIDs, and I/O Systems. 9 Banikazemi, NYU, 2007Latency & Bandwidth Improvements In the time that the disk bandwidth doubles the latencyimproves by a factor of only 1.2 to 1.40204060801001983 1990 1994 1998 2003Year of IntroductionBandwidth (MB/s)Latency (msec)G22.2233 L13 Disks, RAIDs, and I/O Systems. 10 Banikazemi, NYU, 2007Aside: Media Bandwidth/Latency Demands Bandwidth requirementsO High quality video- Digital data = (30 frames/s) × (640 x 480 pixels) × (24-b color/pixel) = 221 Mb/s (27.625 MB/s)O High quality audio- Digital data = (44,100 audio samples/s) × (16-b audio samples) ×(2 audio channels for stereo) = 1.4 Mb/s (0.175 MB/s)O Compression reduces the bandwidth requirements considerably Latency issuesO How sensitive is your eye (ear) to variations in video (audio) rates?O How can you ensure a constant rate of delivery?O How important is synchronizing the audio and video streams?- 15 to 20 ms early to 30 to 40 ms late is tolerableG22.2233 L13 Disks, RAIDs, and I/O Systems. 11 Banikazemi, NYU, 2007Dependability, Reliability, Availability Reliability – measured by the mean time to failure(MTTF). Service interruption is measured by mean time to repair (MTTR) Availability – a measure of service accomplishmentAvailability = MTTF/(MTTF + MTTR) To increase MTTF, either improve the quality of the components or design the system to continue operating in the presence of faulty components1. Fault avoidance: preventing fault occurrence by construction2. Fault tolerance: using redundancy to correct or bypass faulty components (hardware)O Fault detection versus fault correctionO Permanent faults versus transient faultsG22.2233 L13 Disks, RAIDs, and I/O Systems. 12 Banikazemi, NYU, 2007RAIDs: Disk Arrays Arrays of small and inexpensive disksO Increase potential throughput by having many disk drives- Data is spread over multiple disk- Multiple accesses are made to several disks at a timeReliability is lower than a single disk But availability can be improved by adding redundant disks (RAID)O Lost information can be reconstructed from redundant informationO