A presentation at 2nd Conference on File and Storage Technology (FAST) in in San Francisco, CA, USA by erik riedel
SCSI vs. ATA – More than an interface Dave Anderson, Jim Dykes, Erik Riedel Seagate Research April 2003
Outline Marketplace • personal – desktop, low-end servers • enterprise – servers, workstations, arrays • consumer – appliances • mobile – laptops Mechanics & Electronics • choices, comparison Performance • the direct impact Reliability • factors Summary SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Disc characteristics User-visible characteristics • Data rate ~ (2πr) x (density) x (rpm) • Capacity ~ (πr2) x (density) • Seek time ~ r Internal characteristics • Areal density – density • Platter size – r • Spindle speed – rpm SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Areal density higher density, higher data rate Data rate ~ (2πr) x (density) x (rpm) Capacity ~ (πr2) x (density) Seek time ~ r higher density, higher capacity Areal density – density • how many bits you can squeeze in The bad news • requires more signal processing • tolerates less “noise” • harder to do track-following (servo) SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Platter size larger platter, higher data rate Data rate ~ (2πr) x (density) x (rpm) Capacity ~ (πr2) x (density) larger platter, higher capacity Seek time ~ r Platter size – r • large, smaller, smallest bad news – larger platter, higher seek time More bad news • larger platter, more power SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Spindle speed faster platter, higher data rate Data rate ~ (2πr) x (density) x (rpm) Capacity ~ (πr2) x (density) Seek time ~ r Spindle speed – rpm • slow, fast, very fast The bad news • creates more “noise” • more (bad) vibration • more speed, more power SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Comparing mechanics Cheetah 10K.6 enterprise Barracuda 7200.7 personal 84 mm more rigid structure larger actuator assembly SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology 95 mm
Comparing mechanics (2) Cheetah 10K.6 enterprise Cheetah 15K.3 enterprise 84 mm 65 mm smaller platters – less mass, shorter seeks => less capacity SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Mechanics summary Basic design choices • how high data rate • how much capacity • how small seek time Each one affects which parts you pull off the shelf • how you put them together depends on how they will be used • some decisions driven by the marketplace • some driven by cost SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Outline Marketplace • personal • enterprise Mechanics • choices – pick your parts • the choices made to date – comparison Electronics • comparison Performance • the direct impact Summary SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Spindle speed 18000 personal enterprise 12000 9000 6000 3000 20 02 20 00 19 98 19 96 19 94 19 92 19 90 0 19 88 • when it becomes cheap enough, it becomes the norm in personal drives as well 15000 rpm • highest spindle speed will be perfected in enterprise drives product information for Seagate and Control Data disc drives since 1988 SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Capacity 1000000 • this can confuse comparisons based on equivalent capacity 100000 megabytes • wider spacing for enterprise drives • wait until you get 2x 10000 1000 100 personal 10 enterprise 20 03 20 01 19 99 19 97 19 95 19 93 • highest capacity was always in enterprise drives, until 2003 19 91 1 product information for Seagate disc drives introduced since 1991 SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Seek time 100 SCSI vs. ATA at FAST 2003 April 2003 personal enterprise 20 03 20 01 19 99 19 97 19 95 1 19 93 • sensitive to both the mechanics and the signal processing • moving the arm fast enough (starting) • staying on track (stopping) 10 19 91 • trend is toward further separation ms • enterprise always more aggressive on seek performance product information for Seagate disc drives introduced since 1991 © Copyright 2003 Seagate Technology
Rotational vibration 200 150 IOPS • rotation of one drive affects neighboring drives • personal drives not designed to reject the external energy • manifests itself as a performance problem enterprise personal 40% 80% 100 50 SCSI vs. ATA at FAST 2003 April 2003 vibration (rad/s2) comparison of Cheetah 18LP and Barracuda III © Copyright 2003 Seagate Technology 53 42 32 21 11 • state of the art cabinets have 15 adjacent drives • measurement of cabinets shows vibration up to 45 rad/s2 (best cabinets at 5 rad/s2) 0 0
Outline Marketplace • personal • enterprise Mechanics • choices & comparison Electronics • this is the only place where interface matters directly Performance • the direct impact Summary SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Electronics comparison Servo processor – track following • more tracks require more processing • enterprise drives will use two processors Data mover ASIC – for all common-case data transfer • more complex interface requires more gates Program & data memory – every KB counts here! • more complex interface requires more RAM & flash • command queuing, multi-host support, parallel tasks • requires more code, more data space • we only put in the functions users need and use SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Command queuing 250 200 seek time difference IOPS 150 queuing benefit 100 personal enterprise 50 • enterprise has a much larger improvement due to more sophisticated queuing • requires more memory • and more code read 250 • note – queuing benefit on writes would be even larger without (unsafe) caching 32 queue depth 16 8 4 2 1 0 write cache enabled IOPS 200 150 queuing benefit 100 personal 50 SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology 32 queue depth 16 8 4 write 2 0 1 comparison of Cheetah 73LP and Barracuda IV enterprise
Outline Marketplace • personal • enterprise Mechanics • choices & comparison Electronics • this is the only place where interface matters directly Performance • the direct impact Summary SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Performance comparison cap rpm seek density UltraStar 36 LZX SCSI 36 GB 10000 4.9 ms 7.0 Gb/in2 DeskStar 75 ATA 30 GB slower spindle p ext bw 84 mm 6 36 MB/s 8.5 ms 11.0 Gb/in2 95 mm 2 37 MB/s higher areal personal bandwidth density slightly higher equivalent areal larger enterprise bandwidth platters density much higher cap UltraStar 36Z15 7200 dia rpm seek density dia p ext bw SCSI 36 GB 15000 4.1 ms 10.7 Gb/in2 65 mm 6 53 MB/s faster spindle SCSI vs. ATA at FAST 2003 April 2003 faster seek smaller platters more platters © Copyright 2003 Seagate Technology
Summary and conclusions Drives are designed from the ground up to meet a specific set of usage characteristics • more sophisticated than just $ / GB If you want to understand the state-of-the-art • make sure you look at enterprise drives • and make sure you are comparing apples to apples There is room for a wider variety of models • tell us what points are worthwhile SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
More Details? Just ask. dave.b.anderson@seagate.com james.e.dykes@seagate.com erik.riedel@seagate.com SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Detail Slides SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Disc Drive Internals SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Number of platters single head single platter three heads 3-4 platters 5-6 platters 6+ platters two platters 100% • trend toward depopulated drives, as users trade capacity for performance and reliability 80% • even to odd-numbered head and single head drives, to save head costs 20% 60% 40% 20 02 20 01 20 00 19 99 19 98 0% product information for entire industry since 1998 SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Areal density vs. linear density 1000 • most of the recent improvement due to higher tracks per inch (tpi) • linear density, bits per inch (bpi) is much higher, but growing more slowly 100 10 density kbpi ktpi 20 03 20 02 20 01 20 00 19 99 1 product information from the Appendix of “SCSI vs. ATA – more than an interface” in 2nd FAST Conference, April 2003 SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
Linear density 1000 100 kbpi • enterprise lags personal by a little • takes longer to get the signal processing into the right tolerances 10 personal enterprise 20 03 20 01 19 99 19 97 19 95 19 93 1 19 91 • data rate and capacity are the drivers in personal product information for Seagate disc drives introduced since 1991 SCSI vs. ATA at FAST 2003 April 2003 © Copyright 2003 Seagate Technology
This paper sets out to clear up a misconception prominent in the storage community today, that SCSI disc drives and IDE (ATA) disc drives are the same technology internally, and differ only in their external interface and in their suggested retail price. The two classes of drives represent two different product lines aimed at two different markets. In fact, both classes contain a range of products that address a variety of features and usage patterns beyond simply the interface used to talk to the device. The target market and final product specification are taken into account from the earliest design decision through the manufacturing and testing process. This paper attempts to clarify the differences by illuminating some of these design choices and their consequences on final device characteristics. This will hopefully allow the community to build better storage systems with better knowledge of the trade-offs being made and the performance characteristics that result.
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