We have a PostgreSQL 11.5.6 database running on VM.
RAM - 48GB
CPU - 6 cores
Disk - SSD on SAN
We wanted to check how the WAL disk is performing using pg_test_fsync.We ran a test and got around 870 ops/sec for opendatasync and fdatasync and just 430 ops/sec for fsync.We feel it is quite low as compared to what we get for local storage(2000 ops/sec for fsync).What is the recommended value for fsync ops/sec for PosgreSQL WAL disks on SAN ?
Test Results:
pg_test_fsync -f /WAL/pg_wal/test -s 120
120 seconds per test
O_DIRECT supported on this platform for open_datasync and open_sync.
Compare file sync methods using one 8kB write:
(in wal_sync_method preference order, except fdatasync is Linux's default) open_datasync 877.891 ops/sec 1139 usecs/op fdatasync 880.911 ops/sec 1135 usecs/op fsync 433.456 ops/sec 2307 usecs/op fsync_writethrough n/a open_sync 450.094 ops/sec 2222 usecs/op
Compare file sync methods using two 8kB writes:
(in wal_sync_method preference order, except fdatasync is Linux's default) open_datasync 439.119 ops/sec 2277 usecs/op fdatasync 898.221 ops/sec 1113 usecs/op fsync 456.887 ops/sec 2189 usecs/op fsync_writethrough n/a open_sync 229.973 ops/sec 4348 usecs/op
Compare open_sync with different write sizes:
(This is designed to compare the cost of writing 16kB in different write
open_sync sizes.) 1 * 16kB open_sync write 453.444 ops/sec 2205 usecs/op 2 * 8kB open_sync writes 223.142 ops/sec 4481 usecs/op 4 * 4kB open_sync writes 116.360 ops/sec 8594 usecs/op 8 * 2kB open_sync writes 55.718 ops/sec 17948 usecs/op 16 * 1kB open_sync writes 27.766 ops/sec 36015 usecs/op
Test if fsync on non-write file descriptor is honored:
(If the times are similar, fsync() can sync data written on a different
descriptor.) write, fsync, close 445.493 ops/sec 2245 usecs/op write, close, fsync 448.196 ops/sec 2231 usecs/op
Non-sync'ed 8kB writes: write 132410.061 ops/sec 8 usecs/op
