> So the clog is not written to every time the xlog is written to?

No.  One clog page holds 32000 transactions' worth of transaction status
values, so on average we need only one clog page write per 32000

> On a related issue, what's the connection between the "fsync" and the
> "wal_sync_method" configuration switches?

fsync is the master circuit breaker: turn it off, there is no power at
the wal_sync_method socket ;-).  We stop doing anything special about
enforcing write ordering for any files, but just assume that the kernel
+ hardware can be trusted not to lose data they've been given.

With fsync on, wal_sync_method means something.

fsync on also enables fsync/sync for the data files at checkpoint
times.  We don't need to force writes for those between checkpoints,
so wal_sync_method doesn't apply to data files (nor clog).

                        regards, tom lane

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