James's Blog

Oracle: i/o wait events

1) db file sequential read:
single block read into one SGA buffer
single block I/Os are usually the result of using indexes
rarely, full table scan calls could get truncated to a single block call due to
extent boundaries, or buffers already present into the buffer cache.

2) db file scattered read:
multiblock read into many discontinuous SGA buffers
scattered read wait event identified that a full table scan is occurring.

3) direct read:
single or multiblock read into the PGA, bypassing the SGA

posted by James Pei @ 5:04 PM