pg_test_fsync is intended to give you a reasonable idea of what the fastest 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 difference in real database throughput, especially since many database servers are not speed-limited by their transaction logs.
pg_test_fsync accepts the following command-line options:
Specifies the file name to write test data in. This file should be in the same file system that the pg_xlog directory is or will be placed in. (pg_xlog contains the WAL files.) The default is pg_test_fsync.out in the current directory.
Specifies the number of operations per test. The more operations per test, the greater the test's accuracy, but the longer it takes to run. The default is 2000.