Five-Minute Rule for trading memory for disc access-Jim Gray and G. F Five-Minute Rule for trading memory for disc access-Jim Gray and G.F.Putzolu Aashish Grover (MT15002) Ashish Kumar Garg (MT15010) Jyoti Shukla (MT15022) Rajat Tripathy (MT15050)

Outline Problem Statement The Five-minute rule Illustrations 1986 Five-minute rule 10 years later Illustrations 1997 Key Ideas One-minute sequential rule Why One-minute rule Summary

Problem Statement For the earlier systems, memory size was low and cost per byte was high. So the question arises: When it is economic to place the data in main memory rather than to place it in disc(secondary memory). In some situations, response time also dictates the data should be main memory resident.

The Five-Minute Rule For the systems of the 1980’s the solution is “Pages referenced every five minutes should be memory resident”. This introduces a term “Break-even point” -a point where no profit no loss. The break-even reference interval defined as: RI(in seconds)=

Illustrations – In 1986 For Tandem Systems: -Tandem disc delivers 15 accesses/sec and priced 15K$ (for 180Mb). -Tandem main memory costs 5K$ for 1MB size. For the above system, break-even point for 1KB record is an access every 400sec. Any 1KB record accessed more frequently than every 400 sec should live in main memory. These 400sec is approximated to 5 minutes. Hence the name came: “Five Minute Rule”

Contd… For the Tandem System : Formal derivation and statement: RI: expected interval in seconds between references to the page. M$: be the cost of a byte of main memory ($/byte) A$: be the cost of a disc access per second ($/a/s) B: The size of the record/data to be stored in bytes. Bmax: be the maximum transfer size of the disc in bytes

Contd…. For smaller records, break-even interval is longer and shorter for large records.

Five minute rule 10 years later For 1997’s technology, -five minutes is a good lifetime for randomly accessed pages. -one minute is a good lifetime for two pass sequentially accessed pages. These rules change as technology ratios change. RI(in seconds)=

Illustrations – In 1997 For a system with following characteristics -Pages Per MB of RAM = 128 pages/MB (8KB pages) -Accesses Per Second Per Disk = 64 access/sec/disk -Price Per Disk Drive = 2000 $/disk (9GB + controller) -Price Per MB of DRAM = 15 $/MB_DRAM Reference interval calculated as 266 sec ~ 5 minutes

Key Ideas Different rates at which parameters changes -Seek/second & Disk capacity – (512/30 to 128/64 ) -Disk prices dropped 10 times and RAM prices dropped 200 times – (20K$/2K$ to 2K$/15$)

Contd… Above calculation shows that the reference interval is almost unchanged when the technology ratio and economic ratio changes. For DELL TPC-C it comes out 266sec. For TPC-C Compaq having 3 times higher RAM price and 1.5 higher disc price it gives 2 minutes rule. The reference interval varies from one minute to ten minute. Hence in 1997 the five minute rule is applicable to random access pages.

One minute sequential rule One pass algorithms -reads data and never references it. -no need to cache the data in RAM. -system needs only enough buffer memory to allow data to stream from disk to main memory. -Typically, two or three one-track buffers (~100 KB) are adequate per disk to buffer disk operations and allow the device to stream data to the application.

Contd… Two pass algorithms -sequential operations that read a large dataset and then revisit parts of the data. -Database join, cube, rollup, and sort operators -Sorting uses two pass if memory size is smaller than the data set size -Inter reference time is typically about a minute (sequential data access)

Why one minute rule… For DEC TPC bench mark with following specification -Pages per MB of RAM = 16 -accesses per second per disk = 80 -price per disk drive = 2000$ -price per MB of RAM = 15$ The reference interval is calculated as 26 sec. Actually sort would double the IO cost to 52 sec. “One minute rule ”

Summary Five minute rule still applies to randomly access pages(of size 1KB to 8KB). For large pages and sequential access, one minute rule applies.

Thank You