[linux-lvm] poor read performance on rbd+LVM, LVM overload

[Christoph Hellwig]

On Mon, Oct 21, 2013 at 11:01:29AM -0400, Mike Snitzer wrote:
> It isn't DM that splits the IO into 4K chunks; it is the VM subsystem
> no?

Well, it's the block layer based on what DM tells it.  Take a look at
dm_merge_bvec

>From dm_merge_bvec:

	/*
         * If the target doesn't support merge method and some of the devices
         * provided their merge_bvec method (we know this by looking at
         * queue_max_hw_sectors), then we can't allow bios with multiple vector
         * entries.  So always set max_size to 0, and the code below allows
         * just one page.
         */
	
Although it's not the general case, just if the driver has a
merge_bvec method.  But this happens if you using DM ontop of MD where I
saw it aswell as on rbd, which is why it's correct in this context, too.

Sorry for over generalizing a bit.