db file scattered read is a wait event. It's a multiblock read into many discontinuous SGA buffers
This event signifies that the user process is reading buffers into the SGA buffer cache and is waiting for a physical I/O call to return. A db file scattered read issues a scattered read to read the data into multiple discontinuous memory locations.
A scattered read is usually a multiblock read.
It can occur for a fast full scan (of an index) in addition to a full table scan.
The db file scattered read wait event identifies that a full scan is occurring.
When performing a full scan into the buffer cache, the blocks read are read into memory locations that are not physically adjacent to each other.
Such reads are called scattered read calls, because the blocks are scattered throughout memory.
Multiblock (up to DB_FILE_MULTIBLOCK_READ_COUNT blocks) reads due to full scans into the buffer cache show up as waits for 'db file scattered read'.
Check the following VSESSION_WAIT parameter columns:
Figure below depicts the differences between the following wait events:
On a healthy system, physical read waits should be the biggest waits after the idle waits. However, also consider whether there are:
Other things that could indicate excessive I/O load on the system include the following: