Hi, I am currently exploring the pg_start_backup() and pg_stop_backup() functions.
In the documentation (https://www.postgresql.org/docs/9.0/functions-admin.html), it is stated that after calling pg_stop_backup() Postgres switches to the new WAL segment file. But it doesn’t say the same for pg_start_backup(). However, I found the following comment regarding pg_start_backup() in the source code: Excerpt from Postgres source code https://doxygen.postgresql.org/xlog_8c_source.html#l10595 * Force an XLOG file switch before the checkpoint, to ensure that the * WAL segment the checkpoint is written to doesn't contain pages with * old timeline IDs. That would otherwise happen if you called * pg_start_backup() right after restoring from a PITR archive: the * first WAL segment containing the startup checkpoint has pages in * the beginning with the old timeline ID. That can cause trouble at * recovery: we won't have a history file covering the old timeline if * pg_wal directory was not included in the base backup and the WAL * archive was cleared too before starting the backup. So does it mean that Postgres always switches to the new WAL segment file on pg_start_backup() call too? If so, as I understood, the newly created WAL segment file should start from the checkpoint and should not contain any WAL records regarding the events that happened before pg_start_backup() call? Thanks, Daniil Zakhlystov
