A server process is waiting on “db file scattered read” wait event after it performs a multiblock I/O operation and it is waiting for the operating system to complete it.

A multiblock I/O operation occurs when the server process asks for a “chunk” of blocks and not for a single block. The amount of blocks in the “chunk” is determined by the DB_FILE_MULTIBLOCK_READ_COUNT initialization parameter. For example, if the database block size is 8k and DB_FILE_MULTIBLOCK_READ_COUNT is 8, a single multiblock I/O operation will read 8 blocks, which is 64KB of data. The maximum size of a multiblock I/O operation is determined by the operating system and storage.

When talking about performance, there are several things we need to remember:

• A server process can perform. multiblock I/O operations only in certain cases like full table scans and fast full index scans. It is important to remember that multiblock I/O and DB_FILE_MULTIBLOCK_READ_COUNT are irrelevant to other operations such as index  range/unique scans.
• Setting the DB_FILE_MULTIBLOCK_READ_COUNT to a high value will result in less I/O operations (as long as it is not exceeding the operating system and storage limitation).
• Setting the DB_FILE_MULTIBLOCK_READ_COUNT to a high value can also result in unwanted full table scans, since a higher value lowers the optimizer cost of full table scans.
• Starting in 10gR2, Oracle will determine the best value for the DB_FILE_MULTIBLOCK_READ_COUNT parameter if it is not set. If the parameter is set, Oracle will not automatically set its value.