Tom Lane wrote: > Bruce Momjian <pgman@candle.pha.pa.us> writes: > > This is a much simpler fix than people talking about writing their own > > backup programs. > > Well, it's still not exactly trivial. The hack that was being proposed > involved having the admin manually do > > full_page_writes = ON (ie, edit config file and SIGHUP) > pg_start_backup > take backup dump > pg_stop_backup > full_page_writes = OFF (ie, edit config file and SIGHUP) > > with some additions to pg_start_backup/pg_stop_backup to complain if > full_page_writes isn't ON. Aside from being a PITA, this isn't at > all secure, first for the obvious reason that we're only checking > full_page_writes at start/stop and not whether it was on for the whole > interval, and second because SIGHUP is asynchronous. Backends respond > to the signal when they feel like it (in practice, upon starting a new > interactive command) and so it'd be quite possible for a long-running > query to still be executing with full_page_writes off long after the > pg_start_backup has occurred. > > If we were to do this, I'd want some more-bulletproof mechanism for > forcing full_page_writes on during the backup. We could probably > keep a "backup in progress" flag in shared memory, and examine that > along with the GUC variable before deciding to omit a full-page write. > > I seem to recall that there were previous proposals for such a flag, > which I resisted because I didn't want any macroscopic user-visible > change in behavior during a backup. But forcing full-page WAL writes > is something I could live with as a "backup mode" behavior.
Yes, good point. The setting has to be seen by all backends at the same time, so yea, a shared memory variable seems required. The manual method is clearly a loser. -- Bruce Momjian http://candle.pha.pa.us EnterpriseDB http://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---------------------------(end of broadcast)--------------------------- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
