For optimal disk performance, Oracle should always use direct I/O
to its data files, bypassing any caching at the OS layer. Direct
I/O must be enabled both in Oracle via the filesystemio_options
IO must be configured within your operating system.
Oracle controls direct I/O with a parameter named
filesystemio_options. According to the Oracle documentation the
filesystemio_options parameter must be set to SETALL, which
is the preferred method according to the Oracle documentation, or
DIRECTIO in order for Oracle to read data blocks directly from disk.
Also, you must set filesystemio_options = true to use
Oracle external tables.
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