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

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

H&P ch. 7: Storage Systems 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

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.

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

Magnetic Disks 1-12 platters 5000-30,000 tracks/surface 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/write head

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

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

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!

Price/disk vs. year, disk capacity

Price/gigabyte, vs. year

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

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