On Mon, Feb 23 2015 at 1:34pm -0500, Benjamin Marzinski <bmarzins@xxxxxxxxxx> wrote: > On Mon, Feb 23, 2015 at 11:18:36AM -0600, Mike Christie wrote: > > > > If the device/transport is fast or the workload is low, the multipath_busy > > never returns busy, then we can hit Hannes's issue. For 4 paths, we just > > might not be able to fill up the paths and hit the busy check. With only 2 > > paths, we might be slow enough or the workload is heavy enough to keep the > > paths busy and so we hit the busy check and do more merging. > > Netapp is seeing this same issue. It seems like we might want to make > multipath_busy more aggressive about returning busy, which would > probably require multipath tracking the size and frequency of the > requests. If it determines that it's getting a lot of requests that > could have been merged, it could start throttling how fast requests are > getting pulled off the queue, even there underlying paths aren't busy. the ->busy() checks are just an extra check the shouldn't be the primary method for governing the effectiveness of the DM-mpath queue's elevator. I need to get back to basics to appreciate how the existing block layer is able to have an effective elevator regardless of the device's speed. And why isn't request-based DM able to just take advantage of it? (my money is on request-based DM being overly clever but we'll see) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
