I found that bgwriter increments usage count of buffers when it writes
the buffers. I feel this behavior is strange, because the behavior of
bgwriter will affect buffer management strategy.
When backends write LRU dirty buffers, they are replaced immediately.
However, when bgwriter writes dirty buffer, the replacement is postponed
until the next rotation of CLOCK.
If the behavior is not intended, another UnpinBuffer that doesn't
increase the buffers usage count will be needed.
Otherwise, bgwriter should do behavior that looks like VACUUM,
I'll appreciate any comments.
NTT Cyber Space Laboratories
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