Re: How to bypass buffer caches?

From: Helge Hafting (
Date: Fri Feb 14 2003 - 03:36:30 EST

Bruno Diniz de Paula wrote:
> Hi,
> I've sent some messages about using O_DIRECT to read files, but I
> suppose that is not possible using 2.4 kernel and ext2. So I was
> wondering which other alternatives I have to bypass the buffer cache of
> the kernel.

You don't say why you need this. I recommend that you
simply don't use a filesystem - use a partition like
/dev/hda5 without a filesystem and read/write diskblocks
to and from it.

Without a filesystem you decide what data goes in what disk block,
and of course no fs cache gets in the way.

Transfering data between a range of blocks on a partition
and a ordinary file is easy - use the dd command.

dd if=yourfile of=/dev/hdaX bs=4096 seek=<number of first block you want
to use>

dd if=/dev/hdaX of=yourfile bs=4096 skip=<number of first disk block you
want copied> count=<total number of blocks>

Helge Hafting
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