Add a log message on recovery startup before syncing datadir

Hi,

we had a customer incident recently where they needed to do a PITR.
Their data directory is on a NetApp NFS and they have several hundred
databases in their instance. The startup sync (i.e. before the message
"starting archive recovery" appears) took 20 minutes and during the
first try[1] they were wondering what's going on because there is just
one log message ("database system was interrupted; last known up at
...") and the postmaster process is in state 'D'. Attaching strace
revealed that it was syncing files and due to the NFS performance that
took a long time.

I propose to add a message "syncing data directory" before running
SyncDataDirectory() in StartupXLOG() to make that more apparent to the
user, see attached. That should make it clear to users that Postgres is
going to do some work, which, depending on their hardware, could take a
bit.

Thoughts?

Michael

[1] don't ask

--
Michael Banck
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