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

pamvdam at ramoth.xs4all.nl pamvdam at ramoth.xs4all.nl
Mon Oct 4 11:03:12 UTC 1999

On Mon, Oct 04, 1999 at 11:43:27AM +0100, Stephen C. Tweedie wrote:
> 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.

You're right. I did oversee the buffercache pollution issue. Thanks
for pointing it out.

Best regards,


> --Stephen

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