Simon Riggs wrote:
On Sun, 2009-06-21 at 10:28 +0200, Stefan Kaltenbrunner wrote:
I did some limited testing on that but I was unable to measure any
significant effect - especially since the difference between
wal-logged and not is rather small for a non-parallel COPY (ie in the
above example you get around 6m20s runtime for wal-logged and ~5m40s
in the other case).
This is a common confusion for small tests.
Non-WAL logged case causes all buffers to be written to disk at end of
COPY. This is roughly the same size as the volume of WAL written. In
logged case we do not write data blocks, they get written at next
checkpoint. So the reduction in I/O is not apparent, since during the
period of the test the I/O is about the same in both cases and less I/O
in the non-WAL logged case. On longer tests the difference shows more
clearly because the data blocks start to migrate out of shared buffers
while the COPY is still running, effecting the test results.
I was actually testing with and without explicit CHECKPOINTing
before/after the load(and also with longer runs) too - the difference is
negligible especially with only one process involved.
I think the difference is simply not that large because we are still
mostly CPU bound within COPY on reasonably fast IO-subsystems.
Stefan
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