On Tue, May 29, 2012 at 2:12 PM, Kees Jan Koster <kjkos...@gmail.com> wrote: >>> You may want to play around with gshed, the GEOM Scheduler. >>> >>> Matt Dillon did a bunch of tests comparing FreeBSD+UFS to >>> DragonflyBSD+HAMMER and found that FreeBSD starves read threads in >>> order to satisfy write threads (or the other way around?). But, >>> adding gsched into the mix helped things immensely, allowing mixed >>> reads/writes to better shares disk I/O resources. >>> >>> I'll see if I can dig up a link to his testing e-mail messages. >> >> Here's the post, part of a thread on benchmarking RAID controllers: >> >> http://leaf.dragonflybsd.org/mailarchive/kernel/2011-07/msg00034.html > > I looked at "sysctl kern.geom.confdot" (another ridiculously useful feature) > to see where the scheduler should be placed. > > The way I was thinking, I should place a scheduler in such a way that writes > to one physical device (ada3 in my case) do not cause reads on another device > to stall (e.g. ada2, where the database lives). However, it looks like the > GEOM tree is actually a GEOM bush, with a separate tree for each device. > > Am I missing something? Is there a way to schedule across devices? Is the > bush a tree after all, maybe?
There are others much better versed in the ways of GEOM than I, and hopefully they will jump in to simplify/clarify things. :) The way I understand things is that GEOM is a per-device stack of GEOM classes, with the physical device at the bottom, and the VM/block/I/O (?) system at the top. Thus, unless you use one of the multi-device GEOM classes (graid, gmirror, gstripe, gvinum), then each stack is independent of the others. Meaning gsched only works for a single stack (ie, a single device). Granted, I haven't played with gsched yet (most of our high-I/O systems are ZFS), so there may be a way to use it across-GEOMs. -- Freddie Cash fjwc...@gmail.com _______________________________________________ freebsd-stable@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-stable To unsubscribe, send any mail to "freebsd-stable-unsubscr...@freebsd.org"
