Abstract Increases in CPU and memory will be wasted if not matched by similar performance in I/O SLED vs. RAID 5 levels of RAID and respective cost/performance analysis

Background – CPU Performance Unprecedented CPU growth –Gordon Bell: 40% per year between ’74 – ’84 –Bill Joy: Millions of Instructions Per Second = MIPS = 2 Year – 1984 –Mainframes and supercomputers don’t share same growth rate Multiprocessors used to cope with performance

Background – Memory Gene Amdahl: each CPU instruction per second requires one byte of main memory –If system costs are not dominated by the cost of memory the memory chip capacity should grow at same rate Gordon Moore: transistors/chip = 2 Year – 1964 –RAM has quadrupled every 2-3 years Ratio of MB RAM to MIPS has increased in recent years due to dropping memory prices

Background – I/O Primary measure of magnetic disk technology is max number of bits that can be stored per square inch –MAD: maximal areal density –MAD = 10 (Year-1971)/10 Doubled in capacity & halved in price every 3 years

Background – I/O con’t Capacity not the only measure –Performance Main memory increases have kept pace with CPU for 2 reasons –Caches –Speed increases SLED has only increased marginally –Held back because they are mechanical –Seek and rotation delays –Greater density solved only some of problem

Background – I/O con’t Larger main memories and solid state disks as buffers –Problem: volatility High rate of random request for small chunks of data (transaction processing) Low rate of requests for large chunks of data (large simulations on supercomputers)

…the Problem What is the impact of improving the performance of some parts while leaving others the same? Amdahl’s Law: s = 1 _ (1-f) + f/k where: s = effective setup f = fraction of work in faster mode k = speedup while in faster mode

…the Problem con’t Example: –Current applications spend 10% of time in I/O –Using Amdahl’s Law When computers are 10x faster total system speedup will only be 5x When computers are 100x faster, total system speedup will be a mere 10x faster –90% wasted total speedup due to I/O While software & buffering will help in the near term, a solution is needed to avoid crisis

…the Problem con’t Questions: –How can we increase performance of secondary storage and/or disk I/O? –Can a more cost effective solution be found?

…the Solution Arrays of inexpensive disks –PC disks: lower cost and performance –Versus SLED I/Os per second of an inexpensive disk is within a factor of 2 Cheaper per MB, less power consumption –Contains full track buffers and most functions of traditional mainframe controller –Small Computer System Interface (SCSI)

Reliability Unreliable nature of disks forces constant backup MTTF of disk array = MTTF of a single disk Number of disks in array Example: MTTF for 100 CP-3100 disks is 30,000/100 or 300 hours which is less than 2 weeks! Compare to the IBM 3380 which has a MTTF > 30,000 hours Without fault tolerance large arrays of disks are too unreliable to be a viable solution

Better Solution Redundant Arrays of Independent Disks (RAID) Things to consider –Reliability –Overhead cost –Usable storage percentage –Performance

Reliability Regarding reliability –Check disks with redundant information –Replacement of disk occurs in short time (MTTR) Obsolescence –Extremely high MTTF is ‘overkill’ –Are you still using a 20 year old disk?

Overhead Cost Number of extra check disks

Usable Storage Percentage Percentage of total disk space allocated to actual data Another way of viewing costs of overhead

Performance Various applications have various uses for the I/O –Supercomputers: number of reads/writes per second on large chunks of data –Transaction processing: number of individual read/writes per second Also use read-modify-write access

Advantages Full data redundancy (reliability) Much lower cost compared to SLEDs Data rate I/O rate Modular growth potential (scalability) Lower power consumption Availability (after failure)

Disadvantages Capacity Quantity of independent disks to achieve size/performance needs Large number of connections/cables

My Opinion Viable solution to problem assuming other issues are resolved –Actual MTTF rates for arrays –Connectivity to 100 or even 1000 disks Low cost allows entry into consumer market Similar solution to dual-core CPUs –Can’t go any faster so just use 2 of them –Growing trend in computer industry

Problem Solved? Increased performance over current secondary storage solutions? –YES More Cost effective than current standard secondary storage solutions? –YES

The End!!! Questions? Comments? Discussion…