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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
