DB file sequential read for Oracle wait event

DB file Sequential read:

Website Explanation:

This event signifies, the user process is reading a buffer to the SGA buffer cache and is waiting for a physical I/Ocall to return. A sequential read is a single-block read.

The single block I/Os is usually the result of the using indexes. Rarely, full table scan calls could get truncated to a single block call because of extent boundaries, or buffers present In the buffer cache. These waits would also show up as db file sequential 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 1)

On a healthy system, physical read waits should is the biggest waits after the idle waits. However, also consider whether there is on db file sequential reads a large data warehouse so should be seeing mostly fu ll table scans with parallel query.

Figure 10-1 depicts the differences between the following wait events:

• dbfile sequential read (single block, read into one SGA buffer)

• dbfile scattered read (Multiblock read into many discontinuous SGA buffers)

• directread (single or multiblock read into the PGA, bypassing the SGA)

Figure 10-1 Scattered read, sequential read, and Direct Path read

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Explain:

DB Sequential Read this event indicates that the user process is reading the buffer into the SGA buffer cache and is waiting for the physical I/O call to return. Sequential reads are single-block reads .

a single block I/O is usually the result of using an index.

full-table scan calls may be truncated by a single block called by the buffer cache for the presence of a range boundary or buffer. These waits will also be displayed as DB file sequential read.

The optimization method for DB file sequential read:

1. From the beginning of reading, increase the size of buffer cache in the SGA to avoid reading from the hard disk each time;

2. Optimization of SQL statements to reduce unnecessary block reads;

This article is from the "Linux Oracle MariaDB" blog, so be sure to keep this source http://wangergui.blog.51cto.com/8504247/1912801

DB file sequential read for Oracle wait event