The quintessence of the problem explained in previous letters: CHECKPOIND command works quite slow, disk write activity is high just after it is executed, and diring the CHECKPOINT SELECT queries work many times slower than usual.
I experimended with bgwriter, but it affects the CHECKPOINT performance very little (possibly 2-3 times faster but no more). What else could I tune?
And the general question - why SELECT queries slowdown during the CHECKPOINT? I thought that Postgres is a version-based database and read queries are never blocked...