Re: fsync data directory after DB crash

On Wed, Jul 19, 2023 at 12:41 PM Michael Paquier <michael(at)paquier(dot)xyz> 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).