Jeff Janes wrote: > It is only a page read if you have to read the page. It would seem optimal > to have bgwriter adventitiously set hint bits and vm bits, because that is > the last point at which the page can be changed without risking that it be > written out twice. At that point, it has been given the maximum amount of > time it can be given for the interested transactions to have committed and > to have aged past the xmin horizon. I seem to recall that the main problem > with that, though, is that you must be attached to a database in order to > determine visibility, and bgwriter is not attached to a database.
Regarding tuple hint bits, I couldn't find any such limitation in SetHintBits, other than in MarkBufferDirtyHint there being some code that would cause trouble: it accesses MyPgXact, which bgwriter would obviously not have. Maybe worth some experimentation ... I'm not sure about vm bits, though. That's a whole different topic. -- Álvaro Herrera http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
