DAP Fall .‘00 ©UCB 1Professor David A. PattersonComputer Science 252Fall 2000DAP Fall .‘00 ©UCB 2• Disk Basics• Disk History• Disk options in 2000• Disk fallacies and performance• Tapes• RAIDDAP Fall .‘00 ©UCB 3• Several platters, with information recorded magnetically on both surfaces(usually)• Actuatormoves head (end of arm,1/surface) over track (“seek”), select surface, wait for sector rotate under head, then read or write– “Cylinder”: all tracks under heads • Bits recorded in tracks, which in turn divided into sectors (e.g., 512 Bytes)PlatterOuterTrackInnerTrackSectorActuatorHeadArmDAP Fall .‘00 ©UCB 4ActuatorArmHeadPlatters (12){SpindleDAP Fall .‘00 ©UCB 5PlatterArmActuatorHeadSectorInnerTrackOuterTrack• Disk Latency = Seek Time + Rotation Time + Transfer Time + Controller Overhead• Seek Time? depends no. tracks move arm, seek speed of disk• Rotation Time? depends on speed disk rotates, how far sector is from head • Transfer Time? depends on data rate (bandwidth) of disk (bit density), size of requestControllerSpindleDAP Fall .‘00 ©UCB 6• Average distance sector from head?• 1/2 time of a rotation– 7200 Revolutions Per Minute ⇒ 120 Rev/sec– 1 revolution = 1/120 sec ⇒ 8.33 milliseconds– 1/2 rotation (revolution) ⇒ 4.16 ms• Average no. tracks move arm?– Sum all possible seek distances from all possible tracks / # possible» Assumes average seek distance is random– Disk industry standard benchmarkDAP Fall .‘00 ©UCB 7• To keep things simple, orginally kept same number of sectors per track– Since outer track longer, lower bits per inch• Competition ⇒ decided to keep BPI the same for all tracks (“constant bit density”)⇒ More capacity per disk⇒ More of sectors per track towards edge⇒ Since disk spins at constant speed, outer tracks have faster data rate• Bandwidth outer track 1.7X inner track!DAP Fall .‘00 ©UCB 8SectorTrackCylinderHeadPlatter• Purpose:– Long-term, nonvolatile storage– Large, inexpensive, slow level in the storage hierarchy• Characteristics:– Seek Time (~8 ms avg)» positional latency» rotational latency•Transfer rate– 10-30 MByte/sec– Blocks• Capacity– Gigabytes– Quadruples every 3 years (aerodynamics)7200 RPM = 120 RPS => 8 ms per revave rot. latency = 4 ms128 sectors per track => 0.25 ms per sector1 KB per sector => 16 MB / sResponse time= Queue + Controller + Seek + Rot + XferService timeDAP Fall .‘00 ©UCB 9• 1956 IBM Ramac — early 1970s Winchester– Developed for mainframe computers, proprietary interfaces– Steady shrink in form factor: 27 in. to 14 in.• 1970s developments– 5.25 inch floppy disk formfactor (microcode into mainframe)– early emergence of industry standard disk interfaces» ST506, SASI, SMD, ESDI• Early 1980s– PCs and first generation workstations• Mid 1980s– Client/server computing – Centralized storage on file server» accelerates disk downsizing: 8 inch to 5.25 inch– Mass market disk drives become a reality» industry standards: SCSI, IPI, IDE» 5.25 inch drives for standalone PCs, End of proprietary interfacesDAP Fall .‘00 ©UCB 10Data densityMbit/sq. in.Capacity ofUnit ShownMegabytes1973:1. 7 Mbit/sq. in140 MBytes1979:7. 7 Mbit/sq. in2,300 MBytessource: New York Times, 2/23/98, page C3, “Makers of disk drives crowd even mroe data into even smaller spaces”DAP Fall .‘00 ©UCB 11• Late 1980s/Early 1990s:– Laptops, notebooks, (palmtops)– 3.5 inch, 2.5 inch, (1.8 inch formfactors)– Formfactor plus capacity drives market, not so much performance» Recently Bandwidth improving at 40%/ year– Challenged by DRAM, flash RAM in PCMCIA cards» still expensive, Intel promises but doesn’t deliver» unattractive MBytes per cubic inch– Optical disk fails on performace but finds niche (CD ROM)DAP Fall .‘00 ©UCB 121989:63 Mbit/sq. in60,000 MBytes1997:1450 Mbit/sq. in2300 MBytessource: New York Times, 2/23/98, page C3, “Makers of disk drives crowd even mroe data into even smaller spaces”1997:3090 Mbit/sq. in8100 MBytesDAP Fall .‘00 ©UCB 13• Capacity+ 100%/year (2X / 1.0 yrs)• Transfer rate (BW)+ 40%/year (2X / 2.0 yrs)• Rotation + Seek time– 8%/ year (1/2 in 10 yrs)• MB/$> 100%/year (2X / <1.5 yrs)Fewer chips + areal densityDAP Fall .‘00 ©UCB 14– 73.4 GB, 3.5 inch disk– 2¢/MB– 10,000 RPM; 3 ms = 1/2 rotation– 11 platters, 22 surfaces– 15,110 cylinders– 7 Gbit/sq. in. areal den– 17 watts (idle)– 0.1 ms controller time– 5.3 ms avg. seek– 50 to 29 MB/s(internal)source: www.ibm.com; www.pricewatch.com; 2/14/00Latency = Queuing Time + Controller time +Seek Time + Rotation Time + Size / Bandwidthper accessper byte{+SectorTrackCylinderHeadPlatterArmTrack BufferDAP Fall .‘00 ©UCB 15• Calculate time to read 1 sector (512B) for UltraStar72 using advertised performance; sector is on outer trackDisk latency = average seek time + average rotational delay + transfer time + controller overhead= 5.3 ms + 0.5 * 1/(10000 RPM) + 0.5 KB / (50 MB/s) + 0.15 ms = 5.3 ms + 0.5 /(10000 RPM/(60000ms/M)) + 0.5 KB / (50 KB/ms) + 0.15 ms= 5.3 + 3.0 + 0.10 + 0.15 ms = 8.55 msDAP Fall .‘00 ©UCB 16• Bits recorded along a track– Metric is Bits Per Inch (BPI)• Number of tracks per surface– Metric is Tracks Per Inch (TPI)• Care about bit density per unit area– Metric is Bits Per Square Inch– Called Areal Density– Areal Density = BPI x TPIDAP Fall .‘00 ©UCB 17Year Areal Density1973 1.71979 7.71989 631997 30902000 171001101001000100001000001970 1980 1990 2000YearAreal Densit– Areal Density = BPI x TPI– Change slope 30%/yr to 60%/yr about 1991DAP Fall .‘00 ©UCB 180%5%10%15%20%25%30%35%40%45%1974 1980 1986 1992 1998source: New York Times, 2/23/98, page C3, “Makers of disk drives crowd even mroe data into even smaller spaces”470 v. 3000 Mb/si9 v. 22 Mb/si0.2 v. 1.7 Mb/siDAP Fall .‘00 ©UCB 19• Form factor and capacity drives market, more than performance• 1970s: Mainframes ⇒ 14 inch diameter disks• 1980s: Minicomputers, Servers ⇒ 8”, 5.25” diameter disks• Late 1980s/Early 1990s:– Pizzabox PCs ⇒ 3.5 inch diameter disks– Laptops, notebooks ⇒ 2.5 inch disks– Palmtops didn’t use disks, so 1.8 inch diameter disks didn’t make itDAP Fall .‘00 ©UCB 20• 2000 IBM MicroDrive:– 1.7” x 1.4” x 0.2” – 1 GB, 3600 RPM, 5 MB/s, 15 ms seek– Digital camera, PalmPC?• 2006 MicroDrive?• 9 GB,
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