Date: Wed, 22 Jul 2015 16:00:06 -0400 From: Paul Kraus <paul@kraus-haus.org> To: FreeBSD Filesystems <freebsd-fs@freebsd.org> Subject: Re: Prioritize resilvering priority Message-ID: <96FF6F66-06D3-4CAE-ABE5-C608A9A85F7A@kraus-haus.org> In-Reply-To: <CAP1HOmT-qkOf6EuipPs26aNYTPC59_j6CNvK7tubM-HxVJCH-w@mail.gmail.com> References: <CAP1HOmTo28BishnEdPCBsg7V4M4yYfcSKw_AmUbPP-mW4JtRQg@mail.gmail.com> <20150722003218.GD41419@in-addr.com> <CAP1HOmT-qkOf6EuipPs26aNYTPC59_j6CNvK7tubM-HxVJCH-w@mail.gmail.com>
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On Jul 22, 2015, at 14:52, javocado <javocado@gmail.com> wrote: > But I do have: > vfs.zfs.vdev.max_pending: 10 (dynamic) > vfs.zfs.scrub_limit: 10 (loader) > > So, I think I would want to lower one or both of these to increase I/O > responsiveness on the system. Correct? How would the 2 play together in > terms of which to adjust to achieve the best system performance at the > expense of a longer resilver? vfs.zfs.vdev.max_pending is the limit on the number of disk I/O that can be outstanding for a drive (or, IIRC, in this case a given vdev). There has been great debate over tuning this one years ago on the zfs list. The general consensus is that 10 is a good value for modern SATA drives. When I was running 4 SATA drives behind a port multiplier (not a great configuration) I tuned this down to 4 to keep from overwhelming the port multiplier. Tuning it _down_ will reduce overall throughput to a drive. It does not differentiate between production I/O and scrub / resilver I/O. This post: https://forums.freebsd.org/threads/how-to-limit-scrub-bandwidth-vfs-zfs-scrub_limit.31628/ Implies that the vfs.zfs.scrub_limit parameter limits the number of outstanding I/O but just for scrub / resilver operations. I would start by tuning it down to 5 or so and watch carefully with iostat -x to see the effect. Note that newer ZFS code addresses the scrub operation starving the rest of the system from I/O. I have not had a problem on either my FBSD 9 or 10 systems. -- Paul Kraus paul@kraus-haus.org
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