[linux-lvm] LVM, raw-io, devfs

[Stephen C. Tweedie]

Hi,

On Sun, 3 Oct 1999 07:56:44 +0200, pamvdam at ramoth.xs4all.nl said:

> Oracle has always been using filesystems for it's datastorage. Only for
> the parrallel server configuration they demand RAW IO. Like Sybase 11.9.x
> under Linux it probably uses some O_SYNC flag to write to log, so the
> write call only returns when the data has been really written to the
> physical device. I don't think you'll see a performance enhancement when
> using Oracle and RAW/IO. There's even a chance, due to the fact you mis
> the buffer cache optimisations, that you'll see a performance
> degradation.

It depends.  Raw IO uses less CPU --- much less in some cases.  It also
avoids polluting memory with extra copies of data in the buffer cache.
As a result, just measuring the pure disk bandwidth of raw IO versus
buffered IO does not tell you the whole story about performance: you
really need to measure the application's performance itself, including
the effect of CPU and memory pressure.

--Stephen