On Mon, Jun 29, 2020 at 02:56:42PM +0530, Nikhil Shetty wrote: > Hi Team, > > 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 ? Well, it is the VM and SAN overhead, I guess. open_datasync or fdatasync both seem good. --------------------------------------------------------------------------- > > 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 > > > > Thanks and Regards, > Nikhil -- Bruce Momjian <bruce@xxxxxxxxxx> https://momjian.us EnterpriseDB https://enterprisedb.com The usefulness of a cup is in its emptiness, Bruce Lee
