linux-kernel - Doing a zero-copy move of data from a kernel buffer to hard disk

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Date:	Mon, 20 Sep 2010 13:12:22 -0700 (PDT)
From:	Neeraj Kumar <neeraj.kumar01@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Doing a zero-copy move of data from a kernel buffer to hard disk

I am trying to move data from a buffer in kernel space into the hard
disk without having to incur any additional copies from kernel buffer to
user buffers or any other kernel buffers. Any ideas/suggestions would be
most helpful.

The use case is basically a demux driver which collects data into a
demux buffer in kernel space and this buffer has to be emptied
periodically by copying the contents into a FUSE-based partition on the
disk. As the buffer gets full, a user process is signalled which then
determines the sector numbers on the disk the contents need to be copied
to.

I was hoping to mmap the above demux kernel buffer into user address
space and issue a write system call to the raw partition device. But
from what I can see, the this data is being cached by the kernel on its
way to the Hard Disk driver. And so I am assuming that involves
additional copies by the linux kernel.

At this point I am wondering if there is any other mechansim to do this
without involving additional copies by the kernel. I realize this is an
unsual usage scenario for non-embedded environments, but I would
appreciate any feedback on possible options.

BTW - I have tried using O_DIRECT when opening the raw partition, but
the subsequent write call fails if the buffer being passed is the
mmapped buffer.

Thanx!

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