Hi Bruce, Based on pg_test_fsync results, should we choose open_datasync or fdatasync as wal_sync_method? Can we rely on pg_test_fsync for choosing the best wal_sync_method or is there any other way?
Thanks and Regards, Nikhil On Mon, Jun 29, 2020 at 9:36 PM Bruce Momjian <br...@momjian.us> wrote: > 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 <br...@momjian.us> https://momjian.us > EnterpriseDB https://enterprisedb.com > > The usefulness of a cup is in its emptiness, Bruce Lee > >
