John Wren Kennedy c0e58995e3 Large sync writes perform worse with slog ... For synchronous write workloads with large IO sizes, a pool configured with a slog performs worse than one with an embedded zil: sequential_writes 1m sync ios, 16 threads Write IOPS: 1292 438 -66.10% Write Bandwidth: 1323570 448910 -66.08% Write Latency: 12128400 36330970 3.0x sequential_writes 1m sync ios, 32 threads Write IOPS: 1293 430 -66.74% Write Bandwidth: 1324184 441188 -66.68% Write Latency: 24486278 74028536 3.0x The reason is the `zil_slog_bulk` variable. In `zil_lwb_write_open`, if a zil block is greater than 768K, the priority of the write is downgraded from sync to async. Increasing the value allows greater throughput. To select a value for this PR, I ran an fio workload with the following values for `zil_slog_bulk`: zil_slog_bulk KiB/s 1048576 422132 2097152 478935 4194304 533645 8388608 623031 12582912 827158 16777216 1038359 25165824 1142210 33554432 1211472 50331648 1292847 67108864 1308506 100663296 1306821 134217728 1304998 At 64M, the results with a slog are now improved to parity with an embedded zil: sequential_writes 1m sync ios, 16 threads Write IOPS: 438 1288 2.9x Write Bandwidth: 448910 1319062 2.9x Write Latency: 36330970 12163408 -66.52% sequential_writes 1m sync ios, 32 threads Write IOPS: 430 1290 3.0x Write Bandwidth: 441188 1321693 3.0x Write Latency: 74028536 24519698 -66.88% None of the other tests in the performance suite (run with a zil or slog) had a significant change, including the random_write_zil tests, which use multiple datasets. Reviewed-by: Alexander Motin <mav@FreeBSD.org> Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com> Signed-off-by: John Wren Kennedy <john.kennedy@delphix.com> Closes #14378		2023-10-13 11:15:09 -07:00
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spl.4	Add a delay to tearing down threads.	2023-06-26 13:57:12 -07:00
zfs.4	Large sync writes perform worse with slog	2023-10-13 11:15:09 -07:00