And the risks are rather asymmetric. I don't know of any problem from too large a buffer until it starts crowding out shared_buffers, while under-sizing leads to the rather drastic performance consequences of AdvanceXLInsertBuffer having to wait on the WALWriteLock while holding the WALInsertLock,
Suppose you have a large update which generates lots of WAL, some WAL segment switching will take place, and therefore some fsync()s. If wal_buffers is small enough that it fills up during the time it takes to fsync() the previous WAL segment, isn't there a risk that all WAL writes are stopped, waiting for the end of this fsync() ?
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