In chaper 2 of Cost Based Oracle – Fundamentals, I made the following comment about system statistics.
“… you could simply calibrate your hardware (or at least the I/O subsystem) for absolute performance figures …”
I recently received an email asking me how to ‘calibrate the hardware’, and to my surprise I discovered that I had never explained in the book what I meant.
The idea is simple – to my mind, the purpose of system statistics is to tell Oracle how the hardware can behave when under stress; and for most people the most critical hardware is their set of discs themselves.
So my theory is that we would like to tell Oracle about the load that the discs can bear when under pressure. However, for various reasons – including SAN caches, read-ahead algorithms and the like – Oracle loses sight of the discs and can easily gather very misleading statistics about the mreadtim particularly.
To calibrate the devices independently of Oracle, simply create a huge data file (a few hundred gigabytes would be realistic nowadays) and start doing a lot of random reads.
To start with, make the random reads the same size as your Oracle block size, using offsets from the start of file that are always a multiple of the Oracle block size. This is to emulate ramdon indexed access.
Then pick your db_file_multiblock_read_count and change the read size to db_file_multiblock_read_count * db_block_size, still calculating the offsets as a multiple of the single block size. This is to emulate Oracle’s multiblock reads, but avoiding the confusion caused by read-ahead and predictive caching. We do this because the physical I/O still takes place and still impacts on all other I/O activity, even though one individual process may benefit from the read-ahead.
Set the MBRC to your chosen db_file_multiblock_read_count, use the average read time from the first test as the sreadtim, and the average read time from the second test as the mreadtim.
Footnote: in 10g, if you don’t create system statistics, then Oracle “invents” them, based on your db_file_multiblock_read_count and (usually notional) values for the seek time and transfer rate of your disks; and the default values used by Oracle seem to work quite well. If in doubt, setting MBRC to 8, sreadtim to 12 and mreadtim to 26 – the effective defaults for 10g – will probably be better than taking no action in 9i for most OLTP systems.
Warning: System statistics have a global impact on your database. You don’t shouldn’t change them on a production system without going through your usual acceptance test cycle first.
