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Those callbacks were introduced many years ago as part of a bigger patch to smoothen the write throttling within a txg. They allow to account completion of individual physical writes within a logical one, improving cases when some of physical writes complete much sooner than others, gradually opening the write throttle. Few years after that ZFS got allocation throttling, working on a level of logical writes and limiting number of writes queued to vdevs at any point, and so limiting latency distribution between the physical writes and especially writes of multiple copies. The addition of scheduling deadline I proposed in #14925 should further reduce the latency distribution. Grown memory sizes over the past 10 years should also reduce importance of the smoothing. While the use of physdone callback may still in theory provide some smoother throttling, there are cases where we simply can not afford it. Since dirty data accounting is protected by pool-wide lock, in case of 6-wide RAIDZ, for example, it requires us to take it 8 times per logical block write, creating huge lock contention. My tests of this patch show radical reduction of the lock spinning time on workloads when smaller blocks are written to RAIDZ pools, when each of the disks receives 8-16KB chunks, but the total rate reaching 100K+ blocks per second. Same time attempts to measure any write time fluctuations didn't show anything noticeable. While there, remove also io_child_count/io_parent_count counters. They are used only for couple assertions that can be avoided. Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: Alexander Motin <mav@FreeBSD.org> Sponsored by: iXsystems, Inc. Closes #14948 |
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