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 <[email protected]>: > > > On Sun, Nov 1, 2015 at 11:44 PM, Thijs Cramer <[email protected]> > 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 > >
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