synchronous_commit=off doesn't always return immediately

Hi,

I set synchronous_commit to off and expected trivial inserts (single row,
6-8 columns) to always return quickly. However, I see that during
checkpoints they sometimes take over a second. I tried setting the
full_page_write to off, but this didn't seem to have an effect.

The version I'm using is 8.3.3
The configuration is:
bgwriter_delay = 20ms                   # 10-10000ms between rounds
bgwriter_lru_maxpages = 1000            # 0-1000 max buffers written/round
bgwriter_lru_multiplier = 5.0           # 0-10.0 multipler on buffers
scanned/round
synchronous_commit = off                # immediate fsync at commit
full_page_writes = off                  # recover from partial page writes
checkpoint_segments = 32                # in logfile segments, min 1, 16MB
each
checkpoint_timeout = 30min              # range 30s-1h
checkpoint_completion_target = 0.9      # checkpoint target duration, 0.0 -
1.0
checkpoint_warning = 30s                # 0 is off

Thanks,
Tomer
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