On Mon, 21 Oct 2013, Mike Snitzer wrote: > On Mon, Oct 21 2013 at 10:11am -0400, > Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > > > On Sun, Oct 20, 2013 at 08:58:58PM -0700, Sage Weil wrote: > > > It looks like without LVM we're getting 128KB requests (which IIRC is > > > typical), but with LVM it's only 4KB. Unfortunately my memory is a bit > > > fuzzy here, but I seem to recall a property on the request_queue or device > > > that affected this. RBD is currently doing > > > > Unfortunately most device mapper modules still split all I/O into 4k > > chunks before handling them. They rely on the elevator to merge them > > back together down the line, which isn't overly efficient but should at > > least provide larger segments for the common cases. > > It isn't DM that splits the IO into 4K chunks; it is the VM subsystem > no? Unless care is taken to assemble larger bios (higher up the IO > stack, e.g. in XFS), all buffered IO will come to bio-based DM targets > in $PAGE_SIZE granularity. > > I would expect direct IO to before better here because it will make use > of bio_add_page to build up larger IOs. I do know that we regularly see 128 KB requests when we put XFS (or whatever else) directly on top of /dev/rbd*. > Taking a step back, the rbd driver is exposing both the minimum_io_size > and optimal_io_size as 4M. This symmetry will cause XFS to _not_ detect > the exposed limits as striping. Therefore, AFAIK, XFS won't take steps > to respect the limits when it assembles its bios (via bio_add_page). > > Sage, any reason why you don't use traditional raid geomtry based IO > limits?, e.g.: > > minimum_io_size = raid chunk size > optimal_io_size = raid chunk size * N stripes (aka full stripe) We are... by default we stripe 4M chunks across 4M objects. You're suggesting it would actually help to advertise a smaller minimim_io_size (say, 1MB)? This could easily be made tunable. sage _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
 |