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Historically while doing performance testing we've noticed that IOPS can be significantly reduced when all vdevs in the pool are hitting the zfs_mg_fragmentation_threshold percentage. Specifically in a hypothetical pool with two vdevs, what can happen is the following: Vdev A would go above that threshold and only vdev B would be used. Then vdev B would pass that threshold but vdev A would go below it (we've been freeing from A to allocate to B). The allocations would go back and forth utilizing one vdev at a time with IOPS taking a hit. Empirically, we've seen that our vdev selection for allocations is good enough that fragmentation increases uniformly across all vdevs the majority of the time. Thus we set the threshold percentage high enough to avoid hitting the speed bump on pools that are being pushed to the edge. We effectively disable its effect in the majority of the cases but we don't remove (at least for now) just in case we hit any weird behavior in the future. Reviewed-by: George Melikov <mail@gmelikov.ru> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Matt Ahrens <mahrens@delphix.com> Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com> Closes #8859 |
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