Scalus Winter School Storage Systems

Transkript

1 Scalus Winter School Storage Systems Hard Disks André Brinkmann

2 Agenda Hard Disks History Architecture of Hard Disks Disk head, - arm, Calcula?on of access?mes Zoning Trends Influence of TPI, BPI, on performance and price

3 Storage Hierarchy Primary Storage: CPU Register (1 Cycle, ns) Cache ( Cycles, 0,02-0,5µs) Local memory (0,2-4µs) NUMA- memory (2-10 x local memory) Secondary Storage: Hard Disks (2-20 ms) Solid State Disks (0,05-0,5 ms) Cache (0,05-0,5 ms) Ter?ary Storage Exchangeable media(tapes, Floppies, CD,... (ms - minutes) Tape Libraries, op?cal jukeboxes (few seconds - minutes) Mega- Giga Bytes Giga-Tera Bytes Tape libraries (few minutes days) Tera-Peta Bytes Kilo - Mega Bytes

4 History: Punch Card Punch cards Used since middle of 18th century for repea?ng tasks, e.g. in looms or music instruments Hollerith punch card Firstly used in US census in 1890 Standardized in 1928 Used in computer systems since Konrad Zuse s Z1 in 1937 Layout of punch cards has had influence on development of computer languages (Cobol, Fortran) Data processing needs three sets of punch cards: input / output / program Capacity of 80 bytes (80 rows, 12 columns) Punch cards for a pipe organ Slide based on lecture from Prof. R. Burns (Baltimore) / Wikipedia

5 Tapes Tapes are used in computers since 1951 (UNIVAC I) Density of 128 bytes/inch on 8 tracks (6 data tracks/ 1 parity/ 1 clock) Speed of up to 100 in/s Used to store batch applica?ons High latency for random access data Start/stop mode if input stream is too slow Low performance High impact on media life span Today only used as offline backup media LTO drives have performance of up to 140 Mbyte/s (LTO- 5) and uncompressed capacity of 1.5 Tbyte/tape Tape from 1985 Slide based on lecture from Prof. R. Burns (Baltimore) / Wikipedia

6 Tapes Tape durability 15 to 30 years archival 5000 cartridge loads/unloads Approximately 260 full file passes One file pass is equal to wri?ng enough data to fill an en?re tape Oracle StorageTek StreamLine SL8500 Modular Library System Up to 640 tape drives 100,000 slots and 1,000 PB (utilizing 2:1 compression) capacity Slide based on Wikipedia / Oracle

7 History: Hard Disks IBM introduced first hard disk IBM 350 in plalers à 24 Zoll Capacity of 5 MByte Latency of 600 ms Hard disks have been random access memory Hard disks have been small and have been used as main memory, tapes have been persistent memory Today: Hard disks are secondary memory and are moving into backup and archival sector IBM 305 RAMAC, two IBM 350 disks in the foreground and middle Slide based on lecture from Prof. R. Burns (Baltimore) / Wikipedia

8 History: Hard Disks Data density Mbit/sq. in. Capacity of Unit Shown Megabytes 1973: 1. 7 Mbit/sq. in 0.14 GBytes 1979: 7. 7 Mbit/sq. in 2.3 GBytes Source: New York Times, 2/23/98, page C3, Makers of disk drives crowd even more data into even smaller spaces Slide based on lecture from Prof. D. Patterson (Berkeley)

9 History: Hard Disks 1989: 63 Mbit/sq. in 60 GBytes 1997: 1450 Mbit/sq. in 2.3 GBytes 1997: 3090 Mbit/sq. in 8.1 GBytes Source: New York Times, 2/23/98, page C3, Makers of disk drives crowd even more data into even smaller spaces Slide based on lecture from Prof. D. Patterson (Berkeley)

10 History: Hard Disks Evolu?on of hard disks has been driven by form factor and capacity, not by performance 1970s: Mainframes and 14 magne?c disks 1980s: Minicomputers, servers with 8 magne?c disks and 5.25 hard disks 1990s: Pizza box PCs: 3.5 hard disks Laptops, Notebooks: 2.5 hard disks Smaller devices typically do not use hard disks => 1.8 magne?c disks not yet successful Slide based on lecture from Prof. D. Patterson (Berkeley)

11 Seagate DB35 (2006) 750 GB, 3.5 disk 7200 RPM; ATA100 / SATA 9.4 wals (idle) 8.5 ms avg. seek MB/s transfer rate 394 = 0.52 / GB Source:

12 Seagate Barracuda (2009) 1 TB, 3.5- inch disk 7200 RPM; 3 Gbit/s SATA 8 wals idle, 9.6 wals busy? ms avg. seek 120MB/s sustained transfer rate 89 = 0.09 / GB Source:

13 1 Hard disks 2006 Hitachi Travelstar C4K60 71 x 54 x 8 mm 60 GB, 4200 RPM, 15 ms seek 2 plalers, 4 heads Digital cameras, Palm PCs 2006 Hitachi Microdrive 3K8 40 x 30 x 5 mm Weight: 13g 8 GB, 26 MB/s transfer rate Source:

14 Magne?c Disks Purpose in computer systems: Long- term, inexpensive storage for files Backup for main- memory. Large, inexpensive, slow level in the memory hierarchy (virtual memory) Processor Control Data Path Main Memory Input Output Disk Slide based on lecture from Prof. D. Patterson (Berkeley)

15 Photo of Disk Head, Arm, Actuator Arm" Spindle" Actuator" Head" { Platters (12)" Slide based on lecture from Prof. D. Patterson (Berkeley)

16 Disk Device Terminology Arm! Head! Sector! Inner! Track! Outer! Track! Actuator! Platter! Several plalers, with informa?on recorded magne?cally on both surfaces (usually) Bits recorded in tracks, which in turn divided into sectors (e.g., 512 Bytes) Actuator moves head (end of arm) over track ( seek ), wait for sector rotate under head, then read or write Slide based on lecture from Prof. D. Patterson (Berkeley)

17 Read- / Write Heads

18 Recording Technology

19 Perpendicular Recording

20 Disk Drive Performance I Outer Track Inner Track Sector Head Spindle Arm Controller Platter Actuator Disk Latency = Seek Time + Rota?on Time + Transfer Time + Controller Overhead Seek Time? depends on no. tracks to move arm, speed of actuator Rota?on Time? depends on speed disk rotates, how far sector is from head Transfer Time? depends on data rate (bandwidth) of disk (f(bit density,rpm)), size of request Slide based on lecture from Prof. D. Patterson (Berkeley)

21 Disk Drive Performance II Average distance of sector from head? 1/2?me of a rota?on 7200 Revolu?ons Per Minute 120 Rev/sec 1 revolu?on = 1/120 sec 8.33 milliseconds 1/2 rota?on (revolu?on) 4.17 ms Average no. tracks to move arm? Disk industry standard benchmark? Size of Disk cache can strongly affect performance! Cache built into disk system, OS knows nothing Slide based on lecture from Prof. D. Patterson (Berkeley)

22 Seek Time Influence factors on seeks?me Accelera?on of disk arm: Accelera?on between 30-40g Higher weight requires higher forces on disk arm Smaller weight can lead to deforma?on of disk arm è head crash Areal density: Higher TPI and BPI allow smaller diameter at same capacity Smaller disk diameter enable lighter disk arm Higher accelera?on and smaller distances lead to smaller seek?mes Slide based on C. Ruemmler and J. Wilkes: An introduction to disk drive modeling

23 Average Seek Time Average seek?me describes important aspect of access?me Average seek distance is 1/3 of full seek distance (Why?) Different possibili?es to calculate average seek?me 1. Measure?me for access of 1/3 of full seek 2. Divide full seek by 3 3. Sum all?me for all possible seek distances and divide by number of seeks 4. Weight different seek distances from 3. with their access probabili?es Which calcula?on model is best suited to describe average seek?me? Slide based on C. Ruemmler and J. Wilkes: An introduction to disk drive modeling

24 Average Seek Time Warum beträgt die durchschnilliche Seek- Distanz nur 1/3 der maximalen Seek- Distanz? Nehme hierfür an, dass von jedem Track ein Zugriff auf jeden anderen Track erfolgt. Von Track 1 Zugriff auf Track 2,3,,n mit den Distanzen 1,2,,(n- 2), (n- 1) Von Track 2 Zugriff auf Track 3,4,,n mit den Distanzen 1,2,,(n- 2) Von Track (n- 1) nur Zugriff auf Track n mit der Distanz 1 Hieraus kann die folgende Formel gebildet werden: (n-1) mal 2 mal 1 mal n 1 n 1 2 i ( n i) i ( n 1) n (2n 1) i= 1 i= 1 Seek = = n = n 6 = 1 n n 1 n 1 ( n 1) n 3 i i 2 i= 1 i= 1

25 Hilfsmilel Average Seek Time Es gilt für die folgenden Reihen: n i= 1 n ( n+ 1) i = 2 n i= 1 i 2 = n ( n+ 1) (2n+ 1) 6

26 Fallacy Average Seek Time Manufacturer need standard benchmark to compare average seek?me Calculate all seeks from all tracks and divide by number of seeks => average seek?me Real average depends on data layout on hard disk and access palern of the applica?ons => applica?ons ozen access nearby tracks Rule: Measured seek?me typically between ⅓ and ¼ of seek?me on data sheet Slide based on lecture from Prof. D. Patterson (Berkeley)

27 Bestandteile der Seek- Zeit Ein Seek besteht aus Einer Beschleunigungsphase: Der Festplalenarm wird bis zum erreichen des halben Weges oder der maximalen Geschwindigkeit beschleunigt Einer Coast- Phase: Einem Bereich maximaler Geschwindigkeit Einer Verzögerungsphase: Der Festplalenarm wird bis zum Erreichen des Zielgebietes abgebremst Selle- Phase: Der Controller jus?ert den Festplalenkopf Sehr kurze Seeks werden durch Selle- Zeit dominiert (1-3 ms), bzw. bestehen nur aus der Selle- Zeit Kurze Seeks ( Tracks) befinden sich nur in der Beschleunigungsphase Zeit propor?onal zur Wurzel des En~ernung plus der Selle- Zeit Bei langen Seeks bewegt sich der Festplalenarm hauptsächlich mit konstanter Geschwindigkeit Zeit propor?onal zur En~ernung plus einem Overhead Folie basiert auf C. Ruemmler and J. Wilkes: An introduction to disk drive modeling

28 Selle Time Die Feinposi?onierung des Kopfes an dem Ende der Festplalenarmbewegung ist Aufgabe des Track- Following- Systems Reposi?onierung des Festplalenarms auch dann notwendig, wenn nur der Kopf und nicht der Track gewechselt wird (entspricht ca. 1/3 der Selle- Zeit) wenn nach dem Lesen des letzten Sektors eines Tracks zu dem ersten Sektor des folgenden Tracks gesprungen wird (volle Selle- zeit) Mit der Trackdichte steigt die Zeit für die Kopfwechselzeit => geht gegen Selle- Zeit des Festplalenarms Bei Lesezugriffen können Daten bereits vor Beendigung der Feinposi?onierung gelesen werden Geschwindigkeitssteigerung bei Hit Keine Verluste bei Miss Ø Unterschiede der Selle- Zeit für Lesen und Schreiben bis zu 0,75 ms Folie basiert auf C. Ruemmler and J. Wilkes: An introduction to disk drive modeling

29 Einfluss des Datenlayouts Festplale teilt Daten in Sektoren ein Größe zwischen 256 und 1024 Bytes Mapping auf physikalische Sektoren auf der Festplale Bad Sectors können versteckt werden und es kann low- level Performance- Op?mierung durchgeführt werden Zoning: Adjazente Tracks werden in Zonen mit gleicher Anzahl Sektoren eingeteilt (typisch 3 20 Zonen) Track Skewing: Sektor 0 eines jeden Tracks wird verschoben, um die Zeit für den Head- oder Track- Switch zu kompensieren und mit nahezu voller Bandbreite zugreifen zu können Sparing: Es werden Sektoren auf der Festplale reserviert, auf die defekte Sektoren gemapped werden Folie basiert auf C. Ruemmler and J. Wilkes: An introduction to disk drive modeling

30 Zoning: Inner and Outer Tracks Ini?ally: Number of sectors / track the same for inner and outer tracks BPI (Bits per Inch) on outer track smaller than on inner track Technology enables par??oning of disks into zones Each zone chooses number of sectors based on its inner track BPI nearly constant within zone Higher capacity on disk Higher data rate on outer sectors (rpm constant) Performance on outer tracks 1.7-?mes performance on inner track Slide based on lecture from Prof. D. Patterson (Berkeley)

31 Fallacy: Data Transfer Rate Manufacturer provides data rate accessing surface of hard disk on data sheets Each sector contains error correc?ng codes (up to 20% of sector size) and metainforma?on Space between sectors and tracks Rule: Hard disk are slower by a factor of 1,3 compared to data sheets Slide based on lecture from Prof. D. Patterson (Berkeley)

32 Hard Disk Trends Capacity: +100%/year (2x / year) Hard disk capacity has grown that fast that number of plalers per disks has been reduced (some disks only have a single plaler) Transfer rate: +40%/year (2x / 2 year) Rota?on- and seek?me: - 8%/year (1/2 / 10 years) Areal Density Bits per inch, BPI on a track #Tracks on plaler (Tracks per inch, TPI) Important: Bit density per areal unit (Bits/Inch 2 ), called areal density = BPI x TPI MB/ : > 100%/ Jahr (2x / year) Fewer Asics per hard disks and higher areal density Slide based on lecture from Prof. D. Patterson (Berkeley)

33 Hard Disk Capacity

34 Hard Disk Areal Density

35 Hard Disk TPI, BPI,

36 Hard Disk BPI/TPI

37 Magne?c Media Roadmap

38 Hard Disk Roadmap

39 Hard Disk Internal Data Rate

40 Hard Disk Access Times

41 Comparison HDD - DRAM

42 Costs

43 Technology Trends Normalized relative to CPU Network Memory bandwidth Hard disk bandwidth Network latency Hard disk latenxy Slide based on lectur from Prof. R. Burns (Baltimore)

44 Amdahl s Law Amdahl s Law is used to calculate system speed- up if only parts of the system becomes faster Heavily used in parallel compu?ng Generalized Amdahl s Law calculates speed- up S based on S = n 1 k = 0 P S where P k Time frac?on for group S k Speed- up (or slow- down) of group n number of groups 1 k k,

45 Assump?ons: Example for Amdahl s Law 12% of a program can be arbitraily accelerated 88% of the program stay Ques?on: What is the speed- up? S = 0, ,88 1 = 1,136

46 Influence of I/O Performance Relative Ausführungsdauer Jahr Zeit IO Zeit CPU Assump?on: CPU increases 100 %/year, I/O latency decreases by 8% / year Speed- up decreases from 29%/year (1996 à 1997) to 14%/year (1999 à 2000) Slide based on lectures from Prof. D. Patterson (Berkeley) and Prof. R. Burns (Baltimore)

47 Influence of I/O Performance Relative Ausführungsdauer Jahr Zeit IO Zeit CPU Exponen?al performance increase can change cri?cal component Speed- up decreases from 46%/year (1996 à 1997) to 33%/year (1999 à 2000) Slide based on lectures from Prof. D. Patterson (Berkeley) and Prof. R. Burns (Baltimore)