[linux-lvm] poor read performance on rbd+LVM, LVM overload
snitzer at redhat.com
Mon Oct 21 18:27:18 UTC 2013
On Mon, Oct 21 2013 at 2:06pm -0400,
Christoph Hellwig <hch at infradead.org> wrote:
> 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
> >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.
Right, but only if the DM target that is being used doesn't have a
.merge method. I don't think it was ever shared which DM target is in
use here.. but both the linear and stripe DM targets provide a .merge
> Sorry for over generalizing a bit.
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