Re: Concurrent i/o

From: Noons <wizofoz2k_at_yahoo.com.au>
Date: 8 Nov 2005 15:48:17 -0800
Message-ID: <1131493697.191208.146130@g43g2000cwa.googlegroups.com>

Matthias Hoys wrote:

> What do you mean exactly ? I don't see why disabling the UNIX cache would
> require increasing the Oracle buffer cache ? After all, Oracle doesn't
> really read from the UNIX cache, right ? It always uses its own memory
> structures (SGA, PGA), no ?

No, Matthias. Oracle uses the same io calls that every other Unix program
may use. If the particular block it wants to access is not in its buffer
cache, it issues a read() call like any other Unix program. This goes through
whatever io mechanism is available to the file unit the read() points to.
It might or might not use Unix file system cache depending on the Unix cache's capacity, raw disk usage, direct Io usage, io access pattern, etcetc.

Oracle will always use its own memory structures*first*, but not *exclusively*.

What Connor is saying is that if Oracle is set to use the equivalent of

DirectIO, be that as an option to a standard Unix file system or as a result
of concurrent io f/s, then it is advisable to resize the Oracle cache so it
can compensate for not using the Unix file system cache.

And you cannot disable the Unix cache: what you can do is reduce Oracle's
usage of the same by using either a file system that mostly bypasses the Unix cache (concurrent io) or using the DirectIO option for other file systems. Received on Tue Nov 08 2005 - 17:48:17 CST