On Wed, Oct 27, 2010 at 3:53 AM, Simon Riggs <si...@2ndquadrant.com> wrote: > On Tue, 2010-10-26 at 22:03 -0400, Robert Haas wrote: >> On Tue, Oct 26, 2010 at 9:59 PM, Josh Berkus <j...@agliodbs.com> wrote: >> > >> >> If you set wal_keep_segments=0, archive_mode=on, and >> >> archive_command=<something>, you might run out of disk space. >> >> >> >> If you set wal_keep_segments=-1, you might run out of disk space. >> >> >> >> Are you any more screwed in the second case than you are in the first >> >> case? >> > >> > It is the same to the user either way. In either case you have to >> > change some settings and restart the master. >> >> Except that changing wal_keep_segments doesn't require restarting the master. >> >> The point of allowing -1 was to allow someone to set it to that value >> temporarily, to be able to do a hot backup without having to guess how >> large to set it. If you don't have enough disk space for a backup to >> complete, you're kind of hosed either way. > > You're not hosed either way. Fujii designed this carefully to avoid that > and it works. The case of archive_command failing isn't comparable > because that is a failure case, not a normal working server. > > You don't need to guess the setting of wal_keep_segments. It's a safety > net that has been deliberately created to avoid the crash that would > otherwise happen. I've not heard a better proposal, yet, though I too am > hopeful there is a better one.
I think you might be confused about what the use case Bruce and I are imagining, because this doesn't make any sense at all in that context. The specific use case is that you have archive_mode=off, wal_level=archive or wal_level=hot_standby, and you want to take a hot backup. If you do pg_start_backup(), copy the data directory, and do pg_stop_backup(), you won't necessarily end up with enough xlog to reach a consistent state. To do that, you must copy all the WAL files that exist after pg_stop_backup() has completed, but you may not actually be able to do that, because they might get recycled before you can copy them. You can fix this by temporarily increasing max_wal_segments to a sufficiently large value, and then dropping it back down to whatever you like after you've copied the files out of pg_xlog. If -1 isn't an option, you can always use a million, which is almost certainly large enough to be safe. But what you definitely DON'T want to do is temporarily set wal_keep_segments to a value like 7, because if you should manage to go through 8 segments while running the backup, you may find that you now have an unusable backup. And you may not realize this until you try to restore it. I wonder if we should document this procedure. I see that it is not in the docs at present, and it might be useful to somebody. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
