> For the complete documentation index, see [llms.txt](https://docs.postgresql.tw/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.postgresql.tw/15/reference/server-applications/pg_test_fsync.md).

# pg\_test\_fsync

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](/15/server-administration/server-configuration/write-ahead-log.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](/15/server-administration/server-configuration/write-ahead-log.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`.

### See Also

[postgres](/15/reference/server-applications/postgres.md)


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