Re: Raw i/o usage wrecks block device performance??

From: Stephen Tweedie (
Date: Wed Oct 11 2000 - 16:23:52 EST


On Wed, Oct 11, 2000 at 09:28:57PM +0100, Chris Evans wrote:
> The problem is best described with a little sequence. After using raw i/o
> facilities, streamed block device reads from the same underlying device
> exhibit much poorer performance than before the raw i/o.
> Anyone know what's going on? Looks like a bug or inefficiency somewhere in
> the kernel.

It looks as if "hdparm -t" is getting hurt by the fact that raw IO
sets the underlying device's blocksize to 512 bytes (Unix APIs
require that raw character devices allow arbitrary sector-aligned IO).
I'll trace it once I get back from ALS to see if there's anything
obviously funny going on.

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