Thanks Kevin and Ian for your replies. Your information confirms activity I was noticing, but without indepth knowledge of what goes on "behind the current" I was left to speculation - a situation I am extremely uncomfortable with. I have read and re-read the existing documentation on backup and restore for postgresql and I believe we have all of the pieces in place. You both have been a big help. Thanks, David -----Original Message----- From: Kevin Grittner [mailto:Kevin.Grittner@xxxxxxxxxxxx] Sent: Monday, April 11, 2011 10:47 AM To: Ian Lea; David Roland Cc: pgsql-admin@xxxxxxxxxxxxxx Subject: Re: pg_start_backup - backups "David Roland" <david.roland@xxxxxxxxxxxx> wrote: > changes to the data folder continue even after the PG_START_BACKUP > command has been issued. This implies to me that the contents of any > copy of the data folder may be unreliable. i.e. the copy may not > reflect the state of the data folder either before the copy started or > after the copy has finished. It may reflect the state of the data > folder in some transient form. Right. > Assuming this is true, is the copy still usable for restoration? Yes. > If so, how does PostgreSQL get the data folder to a stable state? Is > it by the use of the WAL files that may be created during the backup > process and the restore.config file? Exactly. Simplifying somewhat: - The pg_start_backup causes the WAL position to be remembered. You will need to start WAL replay at this point. - Every significant change to a page is WAL-logged. - Changes after you record the restart point may or may not be in the base backup. - After the base backup copying is complete, pg_stop_backup records the WAL position. You will need to replay WAL *at least* to this point to have a consistent database. - WAL replay will bring every page modified after the "start" to a valid state, whether or not any or all direct modifications to that page made it into the base backup. - This actually goes beyond page fix-ups to file creation and deletion, etc. -Kevin -- Sent via pgsql-admin mailing list (pgsql-admin@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-admin
