I've been pondering the problem myself as I've been planing SMR disk usage for backup ZFS copies.
Is there any difference in resilver process when a mirror is added to a disk in a raidz stripe? My thought was to include a couple enterprise disks of the same capacity for hot sparse then replace them with SMRs after reslivering completes. However, this would only work of ZFS built the mirror linearly. -Chip On Tue, Feb 9, 2016 at 1:01 AM, Thijs Cramer <thijs.cra...@gmail.com> wrote: > To my understanding, a resilver follows the logical order of the blocks in > the ZIO? That means that it will follow (as you suggest) the fragmentation > of the pool. Maybe during a resilver we can read everything 'fragmented', > then write the resilver actions to the ZIL/SLOG, and in a separate action > (thread?) write to the resilvering disks sequentially (if there is enough > space). > This will be greatly beneficial for SMR disks because they can do a fair > amount of random reads, but they lag behind tremendously with random writes > (order of magnitude slower than reads). > > I think it may be to high of a risk to write to the SLOG and then > re-reading it for writing to the resilvering disks. > But the benefits for the resilver process of SMR disks might justify the > risk. > > - Thijs > > 2016-02-08 22:07 GMT+01:00 Matthew Ahrens <mahr...@delphix.com>: > >> >> >> On Sun, Nov 1, 2015 at 11:44 PM, Thijs Cramer <thijs.cra...@gmail.com> >> wrote: >> >>> Hi guys, >>> >>> Recently in a discussion online, i discussed the new Seagate 8TB SMR >>> disks in combination with ZFS. >>> A big point that was undecided was: Can a SLOG help during rebuild of a >>> bit SMR disk based array? >>> >>> I know it's more risky to use the SLOG for resilver activities, but >>> rebuilding sequentially could benefit these disks insanely. The problem >>> with these disks is that under heavy load, the disk because very slow on >>> random writes because of the SMR rewrites the disk has to do.. >>> >>> Can anyone answer whether it's currently beneficial to have a SLOG on an >>> SMR disk array, or answer whether it's easy to adapt the code a bit to make >>> this happen? >>> >>> >> Currently, the ZIL (and thus log devices / SLOG) is not involved in >> resilvering. Therefore adding a SLOG device will have no impact on >> resilver speed. (Though I guess there could be secondary effects if adding >> the SLOG reduces fragmentation in the main devices.) >> >> If you have a specific proposal for how the ZIL / SLOG could be used to >> accelerate resilvering, I can give you feedback on its feasibility. >> >> --matt >> >> > > _______________________________________________ > developer mailing list > developer@open-zfs.org > http://lists.open-zfs.org/mailman/listinfo/developer > >
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