On Sun, Mar 27, 2011 at 2:13 PM, Robert Haas <robertmh...@gmail.com> wrote: > On Mar 26, 2011, at 4:16 PM, Jan Wieck <janwi...@yahoo.com> wrote: >> That was what I meant. Go in steps of 16-64MB backwards and scan from there >> to the current end in forward direction to find a nondeletable block. In >> between these steps, release and reacquire the exclusive lock so that client >> transactions can get their work done. > > Well, VACUUM uses a 16MB ring buffer, so anything that size or smaller should > hit shared_buffers most of the time. > > I wonder though if this might defeat read-behind on operating systems that do > have a working implementation. With our current approach each read will end > at the point the previous read started, which might be an algorithm somebody > is using to detect a backward scan.
Good point. That means the last 16MB of buffers will be in shared_buffers. Anything more than that will definitely not be, because we wrote them out ourselves. So we should truncate in 16MB chunks also. -- Simon Riggs 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
