Re: patch for new feature: Buffer Cache Hibernation

Hi, thanks for good suggestions.

> > 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?

It was my first concern actually. Current implementation is stopping
reading hibernation file when detecting the size mismatch among
shared_buffers and hibernation file. I think it is a safety way.
As Alvaro Herrera mentioned, it would be possible to adjust copying
buffer bloks, but changing shared_buffers setting is not so often I
think.

> How do you protect against the cached buffers getting out-of-sync with
> the actual disk files (especially during recovery scenarios)? What

Saving DB buffer cahce is called at shutdown after finishing
bgwriter's final checkpoint process, so dirty-buffers should not exist
I believe.
For recovery scenarios, I need to research it though...
Could you describe what is need to be consider?

> 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

I think this is important point. I'll implement validation function for
hibernation file.

> 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?

I think this means sequential-read vs scattered-read.
The largest hibernation file is for buffer blocks, and sequential-read
from it would be much faster than scattered-read from database file
via smgrread() block by block.
As Greg Stark suggested, re-reading from database file based on buffer
descriptors was one of implementation candidates (it can reduce
storage consumption for hibernation), but I chose creating buffer
blocks raw image file and reading it for the performance.

Thanks