On Wed, Jul 19, 2023 at 12:41 PM Michael Paquier <michael@xxxxxxxxxxx> wrote: > On Tue, Jul 18, 2023 at 04:50:25PM +0800, Pandora wrote: > > I found that starting from version 9.5, PostgreSQL will do fsync on > > the entire data directory after DB crash. Here's a question: if I > > have FPW = on, why is this step still necessary? > > Yes, see around the call of SyncDataDirectory() in xlog.c: > * - There might be data which we had written, intending to fsync it, but > * which we had not actually fsync'd yet. Therefore, a power failure in > * the near future might cause earlier unflushed writes to be lost, even > * though more recent data written to disk from here on would be > * persisted. To avoid that, fsync the entire data directory. FTR there was some discussion and experimental patches that would add recovery_init_sync_method=none and recovery_init_sync_method=wal, which are based on the OP's observation + an idea for how to make it work even without FPWs enabled: https://www.postgresql.org/message-id/flat/CA%2BhUKGKgj%2BSN6z91nVmOmTv2KYrG7VnAGdTkWdSjbOPghdtooQ%40mail.gmail.com#576caccf21cb6c3e883601fceb28d36b Only recovery_init_sync_method=syncfs actually went in from that thread. It works better for some setups (systems where opening squillions of files just do perform a no-op fsync() is painfully expensive).
