1. Computer Architecture Principles Dr. 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. 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

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 5000-30,000 tracks/surface
RPM
2 recording surfaces ea.
in. diameter
,000 tracks/surface
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

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: 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 !

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: GB
Digital video disk, DVD: 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