John Scalia <jayknowsunix@xxxxxxxxx> wrote: > I've been examining the source code to pg_basebackup written by > Magnus Hagander. It definitely shows that if you do not issue a > "-X f" or "-X s" then the pg_xlogs will NOT be included in the > backup. This leads me to a couple more questions: > > 1) Does anyone know why anything in a WAL segment is required for > a database to start? Yes. This is covered in the docs, but in summary: the files could be modified during the copy, and without WAL replay from before the copy started the database would not necessarily be consistent. > I ask this as the backup_label generated by pg_basebackup > indicates a segment where the latest checkpoint is contained. It indicates the location of the last checkpoint before the start of the backup -- the latest point from which you cab start WAL replay to achieve a consistent database. > Surely, there has to be some better way to permit the DB to > sanely initialize. A write-ahead log is a well-known and widely used technique for this. What would you suggest instead? > 2) If a WAL segment is in fact required for the backed up DB to > start, why would pg_basebackup not include those by default? To > not do so, doesn't create a backup file, just in this case, a > tarball that's worthless. ... unless you are archiving the WAL to somewhere that it will be kept long enough to be usable for such purposes. If you are (and I highly recommend that you do so), including WAL in the base backup is a waste of both bandwidth and storage space. Besides reviewing the documentation's discussions of WAL and backups, you might want to read this: http://tbeitr.blogspot.com/2015/07/deleting-backuplabel-on-restore-will.html -- Kevin Grittner EDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-admin mailing list (pgsql-admin@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-admin
