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. 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) _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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