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File location matters on busy Oracle disks

In this blog entry we see a great test that demonstrates that the placement of data files on different parts of the disk impact the access speed:

So the total number of I/Os that we can do really depends on where the data is located on the disk. According to maxtor the average seek time for this disk is around 9.3 ms. It means that the full seek time will be around 16-18 ms. That seems to fit. Subtract the average outside time from the average inside/out (16.4 - 0.5) and we get 15.9 millisecond as may be 90 percent of the full seek time.

So even for the simple ATA drives, it is important where the data is stored.

The idea is to place frequently-referenced “hot” data files in the middle cylinders of the disk to minimize the “seek delay” associated with the movement of the disk read-write heads.  Back in the 1980’s you could deliberately load hot files on the inner and outer edges of a disk, and the read-write head movement would make the disk shake like an out-of-balance washing machine:

Until such time that solid state disk is cheap enough to fully cache large databases, the Oracle DBA will still be concerned about their most critical performance area, the disk I/O sub-system. The main points of this article include the following:

·         If you are not using RAID, the old-fashion file placement rules still apply, and you must manually place Oracle data files across you disk spindles to relieve I/O contention.

·         Using a RAID 10 approach (striping and mirroring) distributes data blocks across all of your disk spindles, making “hot” disks a random and transient event.

·         RAID 5 is not recommended for high-update Oracle systems. The performance penalty from the parity checking will clobber Oracle performance.

·         Oracle9i and Oracle10g continue to refine the ability to support very large RAM data buffers with the v$db_cache_advice utility and Oracle10g Automatic Memory Management (AMM).

·         Solid state disk is getting cheaper, and may soon replace traditional disk devices. Many Oracle customers are using solid state disk for high I/O data files such as TEMP, UNDO, and REDO files.

·         If disk is not your bottleneck, improving disk access speed will not help. Be sure to check your top-5 STATSPACK wait events to ensure that disk I/O is your bottleneck before undergoing expensive changes to your disk I/O subsystem.


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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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