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

[Neeraj Kumar <neeraj.kumar01@ymail.com>]

On Mon, 20 Sep 2010, 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.

>Isn't this functionality already served by relayfs?

I could not figure out how relayfs could help me.

 >> 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.

>The other zero-copy  mechanism is splice(2).

I looked at the implementation of splice(). It looks promising. I have 2 more 
questions related to it:
1. SPLICE_F_MOVE which is needed for zero copy movement of pages is defined as 
no op in the kernel code.
    Does splice still tries for zero copy movement?

2. I may need to implement splice_read() and splice_write() for the demux 
driver.  I did not find any other
 driver making use of these interfaces in the kernel except file system. Is 
there any sample code available
which could help me to understand/implement splice() interfaces in a better way?


Thanks,
Miklos