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0f740a4f1d
Despite all optimizations, tests on actual hardware show that FreeBSD kernel can't sleep for less then ~2us. Similar tests on Linux show ~50us delay at least from nanosleep() (haven't tested inside kernel). It means that on very fast log device ZIL may not be able to satisfy zfs_commit_timeout_pct block commit timeout, increasing log latency more than desired. Handle that by introduction of zil_min_commit_timeout parameter, specifying minimal timeout value where additional delays to aggregate writes may be skipped. Also skip delays if the LWB is more than 7/8 full, that often happens if I/O sizes are constant and match one of LWB sizes. Both things are applied only if there were no already outstanding log blocks, that may indicate single-threaded workload, that by definition can not benefit from the commit delays. While there, add short time moving average to zl_last_lwb_latency to make it more stable. Tests of single-threaded 4KB writes to NVDIMM SLOG on FreeBSD show IOPS increase by 9% instead of expected 5%. For zfs_commit_timeout_pct of 1 there IOPS increase by 5.5% instead of expected 1%. Reviewed-by: Allan Jude <allan@klarasystems.com> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Prakash Surya <prakash.surya@delphix.com> Signed-off-by: Alexander Motin <mav@FreeBSD.org> Sponsored by: iXsystems, Inc. Closes #14418 |
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