Re: pg_basebackup, walreceiver and wal_sender_timeout

On Sat, Jan 26, 2019 at 01:45:46PM +0100, Magnus Hagander wrote:
> One workaround you could perhaps look at here is to run pg_basebackup
> with --no-sync. That way there will be no fsyncs issued while running. You
> will then of course have to take care of syncing all the files to disk
> after it's done, but a network filesystem might be happier in dealing with
> a large "batch-sync" like that rather than piece-by-piece sync.

Hm.  Aren't we actually wrong in letting the WAL receive method use
the value of do_sync depending on the command line arguments, with
true being the default for pg_basebackup?  In plain format, we flush
the full data directory anyway when the backup ends.  In tar format,
each individual tar file is flushed one-by-one after being received,
and we issue a final sync on the parent directory at the end.  So
what's missing is just to make sure that the fully generated
pg_wal.tar is synced once completed.  This would be way cheaper than
letting the stream process issue syncs for each segments, which does
not matter much in the event of a host crash because the base backup
may finish in an inconsistent state, and one should not use it.
--
Michael