Re: [Qemu-devel] [PATCH 0/4] qcow2: Improve cluster refcount update

[Anthony Liguori <anthony@codemonkey.ws>]

Laurent Vivier wrote:
> This series of patches improves qcow2 performance with O_DIRECT
> when the qcow2 file is empty and we begin to fill it.
>   

I really dislike this series and any future series that does similar 
things.  The real problem is that non-aligned accesses are so slow, but 
there's no reason that they must be so slow.  It's because we're doing 
synchronous IO operations instead of using posix-aio like we should.  
That's the real problem.  Just doing a memory copy to an aligned buffer 
is not going to cause that much performance delay.

We shouldn't be adding more cruft to the code base to avoid fixing the 
real problem.  I think the current implementation of O_DIRECT needs to 
be rewritten.

Regards,

Anthony Liguori