I obtained Oracle for Linux and did some comparisons with PostgreSQL
6.5 current using the test suite I mentioned before that is good to
create heavy loads.
With default postmaster settings (postmaster -S -i), PostgreSQL was
several times slower than Oracle. However with -F (postmaster -S -i -o
'-F'), PostgreSQL was much faster than the default settings. Yes, this
is well known behavior of PostgreSQL. Without -F PostgreSQL does
fsync() every time a transaction is committed, and it is the bottle
neck of the performance. I observed the disk activity LED almost
always on while running PostgreSQL without -F. However with -F, there
may be a chance that we loose committed data if the computer gets
crashed.
On the other hand the LED was on only every few secs while running
Oracle. I heard that Oracle has a "REDO log file" and a log is written
into there when a transaction is committed. If so, apparently Oracle
does not issue sync() or fsync() every time a transaction gets
committed. I don't know how Oracle guarantees the log be written into
the disk without sync() or fsync() at the commit time, but seems
something like it is one of the most important technique to enhance
the performance of PostgreSQL.
Does anybody have an idea on this?
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Tatsuo Ishii