On Tue, Apr 30, 2013 at 3:24 PM, Dmitry Koterov <dmi...@koterov.ru> wrote:
> I think that at > http://www.postgresql.org/docs/current/static/functions-admin.html and > http://www.postgresql.org/docs/current/static/continuous-archiving.html two > important points on how pg_start_backup() works are missing: > > 1. After pg_start_backup() and till pg_stop_backup() VACUUM is denied > (e.g. autovacuum is turned off), so the new data is always appended to data > files, is never written at their middle. > This is not the case. Autovacuum continues to run during the backup. > This allows to archive the data directory using any copying tools (rsync, > tar, cp etc.). If you forget to call pg_stop_backup() by accident, data > files will grow forever. So pg_start_backup() switches the database to > "append-only mode" which is safe to backup without stopping (data files > temporarily become append-only, WAL logs are append-only always). > No, it doesn't work that way. I don't know why appending would be any safer than normal updates would be anyway. WAL replay fixes up any problems that might arise. > 2. After pg_start_backup() and till pg_stop_backup() full_page_writes is > forced to be ON. > Effectively yes, this is documented in one of your links above (and is one of the reasons vacuuming during the backup is not a problem) Cheers, Jeff
