
pg\_test\_fsync — determine fastest `wal_sync_method` for PostgreSQL

## Synopsis

`pg_test_fsync` \[_`option`_...\]

## Description

pg\_test\_fsync is intended to give you a reasonable idea of what the fastest [wal\_sync\_method](../runtime-config-wal.html/index.md#GUC-WAL-SYNC-METHOD) is on your specific system, as well as supplying diagnostic information in the event of an identified I/O problem. However, differences shown by pg\_test\_fsync might not make any significant difference in real database throughput, especially since many database servers are not speed-limited by their write-ahead logs. pg\_test\_fsync reports average file sync operation time in microseconds for each `wal_sync_method`, which can also be used to inform efforts to optimize the value of [commit\_delay](../runtime-config-wal.html/index.md#GUC-COMMIT-DELAY).

## Options

pg\_test\_fsync accepts the following command-line options:

`-f`  
`--filename`

Specifies the file name to write test data in. This file should be in the same file system that the `pg_wal` directory is or will be placed in. (`pg_wal` contains the WAL files.) The default is `pg_test_fsync.out` in the current directory.

`-s`  
`--secs-per-test`

Specifies the number of seconds for each test. The more time per test, the greater the test's accuracy, but the longer it takes to run. The default is 5 seconds, which allows the program to complete in under 2 minutes.

`-V`  
`--version`

Print the pg\_test\_fsync version and exit.

`-?`  
`--help`

Show help about pg\_test\_fsync command line arguments, and exit.

## Environment

The environment variable `PG_COLOR` specifies whether to use color in diagnostic messages. Possible values are `always`, `auto` and `never`.
