Re: patch for new feature: Buffer Cache Hibernation

Mitsuru IWASAKI <iwasaki@jp.FreeBSD.org> writes:
> Postgres usually starts with ZERO buffer cache.  By saving the buffer
> cache data structure into hibernation files just before shutdown, and
> loading them at startup, postgres can start operations with the saved
> buffer cache as the same condition as just before the last shutdown.

This seems like a lot of complication for rather dubious gain.  What
happens when the DBA changes the shared_buffers setting, for instance?
How do you protect against the cached buffers getting out-of-sync with
the actual disk files (especially during recovery scenarios)?  What
about crash-induced corruption in the cache file itself (consider the
not-unlikely possibility that init will kill the database before it's
had time to dump all the buffers during a system shutdown)?  Do you have
any proof that writing out a few GB of buffers and then reading them
back in is actually much cheaper than letting the database re-read the
data from the disk files?
        regards, tom lane