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Oracle db file scattered read

Oracle Consulting Tips by Burleson

 

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

"The db file scattered 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.

A db file scattered read issues a scatter-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 table scan is occurring. When performing a full table 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.

This is why the corresponding wait event is called 'db file scattered read'. Multiblock (up to DB_FILE_MULTIBLOCK_READ_COUNT blocks) reads due to full table scans into the buffer cache show up as waits for 'db file scattered read'."

Oracle guru David Aldridge has noted that "db file sequential reads" (full-scan I/O) can become slower than "db file scattered reads" (single block gets) on Linux.  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).

Oracle read-ahead caching

The read ahead caching has many names on different operating systems, and it started on IBM mainframes as "sequential prefetch".

The concept of readahead caching is simple. Over 90% of I/O latency is consumed in the read-write head movement, as the heads are placed under the target cylinder. Once in-place, the disk continues to spin and the read-write head can simultaneously transmit back the original block request at the same time as the next sequential block passes below the read-write heads.

For complete details on device-level caching, see my book "Oracle Tuning: The Definitive Reference".

For scan operations (index range scans, index fast full scans, and full-table scans), a read-ahead cache can be very useful for speeding up these "scattered read" operations.

In sum, if your database is requesting adjacent data blocks, the read-ahead cache may improve I/O throughput. However, using RAID10 (SAME, stripe and mirror everywhere), like with ASM, the blocks are not adjacent, and a read-ahead cache may not be as useful as a database where the data blocks are laid-out sequentially. Of course, the stripe width influences this decision.

 

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:

Also see these notes on Oracle db file scattered reads:

 


 

 

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