[vdr] Patch to avoid file system buffer trashing

Marko Mäkelä marko.makela at hut.fi
Tue Aug 23 22:24:18 CEST 2005

On Tue, Aug 23, 2005 at 09:00:14PM +0200, Ralf Müller wrote:
> Actually I meant the trouble with programming. It is a difference to 
> align all buffers to 512 bytes and read/write in 512 byte blocks instead 
> of just reading and writing what you want. This is especially true for 
> programs which have not been designed this way from the very beginning. 
> I first started to do a patch based on O_DIRECT - this would have been 
> much more intrusive.

I see, you would need to introduce a buffering layer.

> >Hmm, is there a reason why your WriteStream function doesn't simply do
> >posix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED)?  That would simplify the
> >logic, and if I understood correctly, it should work equally well,
> >except with some early 2.5 kernels.  I understand that you will need to
> >keep track on the offsets in ReadStream because of read-ahead.
> It seemed to me that different streams interfere with each other.  So the 
> posix_fadvise() of a recording thread seemed to kill the read ahead 
> buffer of a player thread for the same recording - very annyoing.

Sorry, I didn't think of that.

> Another nice thing to have would be a way to tell the kernel to forget 
> dirty buffers after they have been written to disc without the need to 
> call fdatasync() before. The forced sync definitly is less then optimal.

In theory, this could be solved by writing a write completion callback
function that would invoke posix_fadvise(), but this asynchronous I/O would
be a real overkill.  As far as I know, aio (asynchronous I/O) is only
available in 2.6 series kernels (and perhaps some Red Hat 2.4 kernels).


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