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Disk Contention

Once DBWR has been tuned, the DBA needs to look at disk contention. Disk contention happens when one or more users attempt to read the same disk at the same time, or in some cases, access a different disk through the same controller path at the same time. Spreading Oracle-related files across several platters or sets of platters--the more the better--prevents this. The new RAID options don’t relieve the DBA of file placement concerns. You should be sure that the RAID volumes are properly set. I had one system where a system administrator set up multiple RAID5 volumes using two disks for each volume (a hint: the 5 is a meaningful number for RAID5).

The report in Source 13.17 can be used to monitor relative-fill I/O efficiency--essentially, how many reads are being performed per request as a percent. The more times the process has to access the same datafile to get the same information, the less efficient the datafile is. This could be caused by colocation of indexes and tables, a poorly ordered table that is scanned frequently, or having temporary or undo segments in with data or index segments.

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