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

[Robert Hancock <hancockrwd@gmail.com>]

On 09/20/2010 02:12 PM, Neeraj Kumar wrote:
> 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.

O_DIRECT would be the right way to do it - I think you'd need to figure 
out why that write is failing. Keep in mind you need to observe the 
alignment restrictions of O_DIRECT (man 2 open).