Re: performance of O_DIRECT on md/lvm

[Andi Kleen <ak@suse.de>]

arjan@fenrus.demon.nl writes:

> In article <p734rlg90ga.fsf@oldwotan.suse.de> you wrote:
> 
> > I think an optional readahead mode for O_DIRECT would be useful. 
> 
> I disagree. O_DIRECT says "do not cache. period. I know what I'm doing"
> and the kernel should respect that imho. After all we have sys_readahead for
> the other part...

Problem with sys_readahead is that it doesn't work for big IO sizes. 
e.g. you read in big blocks. You have to do readahead(next block); 
read(directfd, ..., big-block); 
The readahead comes to early in this case; it would be better if it is
done in the middle of read of big-block based on the request size.
Otherwise you risk additional seeks when you overflow the 'read window',
which is all to easy this way. 

-Andi