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Oracle Metric db file sequential read

Oracle Consulting Tips by Burleson

 

Here are my notes on Oracle db file sequential read disk I/O tuning The Oracle Documentation notes on db file sequential reads:

"The db file sequential read Oracle metric 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.

This call differs from a scattered read, because a sequential read is reading data into contiguous memory space. A sequential read is usually a single-block read.

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 in the buffer cache. These waits would also show up as 'db file sequential read'."

While Oracle guru David Aldridge has noted that Linux kernel 2.6 has four types of I/O scheduling, Completely Fair Queueing (CFQ), Deadline, NOOP, and Anticipatory I/O scheduling.  Under special circumstances when reading large data blocks with ASM, he notes that "db file sequential reads" (single block access) can become slower than "db file scattered reads" (full scan I/O) on Linux, and he notes that full-scan access speed is aggravated by Oracle willy-nilly block placement in Automated Storage Management (ASM) and using bitmap freelists (Automated Segment Storage Management).

Also see these notes on db file sequential reads:

 

 

The popular Ion tool is the easiest way to analyze Oracle cache and disk performance (db block parallel reads and writes), and Ion allows you to spot hidden disk I/O performance trends.  Ion is our favorite Oracle tuning tool, and the only 3rd party tool that we use.  This, and many other Oracle performance metrics are discussed in my book "Oracle Tuning" by Rampant TechPress.  You can buy it directly from the publisher and save 30% at this link:

http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm


 

 

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