DB file scattered read for Oracle wait event

DB file Scattered read

Website Explanation:

This event signifies, the user process is reading buffers to 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, a full scan is occurring. When performing a full scan into the buffer cache, the blocks read is read to memory locations that is not physically Adjacent to all other. Such reads is called scattered read calls, because the blocks is scattered throughout memory. The corresponding wait event is called ' db file scattered read '. 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 V$SESSION_WAIT parameter columns:

• P1: The absolute file number

• P2: The block being read

• P3: The number of blocks (should be greater than 1)

On a healthy system, physical read waits should is the biggest waits after the idle waits. However, also consider whether there is direct read waits (signifying full table scans with parallel query) or DB file sc Attered read waits on a operational (OLTP) system that should be doing small indexed accesses.

Other things this could indicate excessive I/O load on the system include the following:

• Poor Buffer Cache Hit ratio

• These wait events accruing most of the wait time for a user experiencing poor response time

Explanation: DB file scattered read wait event: is due to multi-block read operations, when we retrieve data from the disk to read data into memory, one i/0 read multiple data blocks, and the data block in memory is scattered distribution is not continuous, This process generates a "DB file scattered read" event when the data block is read into memory.

Multiple data block Read scenarios:

(1) FTS (full table Scans) All-table scan

(2) iffs (index fast full Scans) index fast complete Scan: Cut the index chain into many parts, multiple parallel reads.

This article is from the "Linux Oracle MariaDB" blog, so be sure to keep this source http://wangergui.blog.51cto.com/8504247/1912796

DB file scattered read for Oracle wait event