Computer Architecture Principles Dr. Mike Frank

1. Computer Architecture PrinciplesDr. Mike Frank CDA 5155 (UF) / CA 714-R (NTU)Summer 2003 Module #35 Storage Systems

2. Administrivia • Schedule for rest of semester: • Today (Tue.12/3): • Graded proj. #2 returned • Quick overview, chs. 7-8. • Thurs. 12/5: • HW#3 due • HW#3 solutions out • Final exam review session • Tue. 12/10: Last day of class • Return graded HW#3 • Student-teacher evaluations • Final projects due • Final project presentations • Thu.-Fri. 12/12-12/13: Reading days • Thu. 12/19, 7:30 am – 9:30 am: Final Exam

3. Introduction Types of Storage Devs. Buses Reliability, availability, dependability RAID Errors & failures in real systems I/O Performance Measures A Little Queueing Theory Benchmarks of storage perf. & availability. Crosscutting issues Designing an I/O system in 5 easy pieces EMC Symmetrix and Celerra Sanyo VPC-SX500 Digital Camera Fallacies & Pitfalls Conclusion Historical perspective H&P ch. 7: Storage Systems

4. Why Do We Care? About Storage? • Cost of storage can easily dominate the total cost of a whole computer system. • Esp. on data-intensive applications. • Latency and bandwidth of I/O to storage can dominate response time and limit throughput of a computation overall. • Moreso if CPU performance improves faster than I/O, disk • Users are concerned to an extreme degree with the reliability and availability of their storage systems. • Failed CPU → No problem, just go buy another CPU • But, failed storage system → • Possible loss of data which may represent an enormous # of past person-hours of work invested to collect that data • Or, even if backups exist, loss of future labor / opportunity cost, while workers wait for lost data to be restored/recovered.

5. Types of Storage Media • Commonly used today: • Magnetic disks • Magnetic tapes • Automated tape libraries • Optical disks (CDs, DVDs) • Flash memory • Some possible future technologies: • Optical holographic storage • Molecular storage devices

6. Magnetic Disks • 1-12 platters • Spin@3,600-16,000 RPM • 2 recording surfaces ea. • 1.0-3.5 in. diameter • 5000-30,000 tracks/surface • 100-500 sectors/track • Smallest read/write unit • More in outer tracks: • “Constant Bit Density” • Contains overhead: • Sector number, to ID sector • Error correction code • 512 bytes/sector typical Read/writehead

7. Disk Performance Characteristics • Seek time – Time to move arm to desired track • Reported as minimum, maximum, average • Average typically 5-12 ms • Rotation latency, rotational delay – • Time for requested sector to rotate under RW head • Average is ½ of a complete rotation time, e.g.: • 10,000 RPM disk → avg. rotation latency 3.0 ms • Transfer time- • Time to transfer a block (sector), through RW head • Depends on block size, rotation speed, data density, and overhead/BW of disk controller electronics • Typical (ca.2001): 3-65 MB/second

8. Disk Controllers • Often supports asynchronous handling of multiple overlapping requests from CPU • A kind of “pipelining” of disk requests • Analogous to a split-transaction bus to RDRAM • Often  a read-ahead buffer in disk drive unit • Leverages spatial locality in sector references • Typically 1/8 - 4 MB • Transfer rates from buffer: 80-320 MB/s

9. Disk Technology Trends • Areal density (bits/area): • Tracks/inch × bits/inch • -1988: ↑29%/yr; -1996: ↑60%/yr; -2001: ↑100%/yr • 2001: 20 Gb/in2 commercially, 60 Gb/in2 in lab • Cost/GB: dropped inversely as density ↑ • See charts, subsequent slides • Decreased 10,000× from 1983-2000!

10. Price/disk vs. year, disk capacity

11. Price/gigabyte, vs. year

12. The Access Time Gap Cost~100× Latency ~100,000×

13. Optical Disks • Big 12’’ “laser disks” from 80’s: now obsolete • Compact disk, CD: 0.65 GB • Digital video disk, DVD: 4.7-9.4 GB • Read-only: CD-ROM, DVD-ROM • Writable optical technologies: • Write-once: CD-R, DVD-R (“recordable”) • Rewritable: CD-RW, DVD-RW • Read speed usu. ½ that of CD-ROMs • Write speed usu. ¼ CD-ROM read speed