Re: Snapshot backups - Mailing list pgsql-general
I understand what you are saying, and I understand how the backup_label works - but I still don't understand why the pg_start and pg_stop commands are REQUIRED when doing a snapshot backup to ensure data integrity.
Surely not using them and restoring a snapshot is the same as starting after a crash, and will result in log replay to get to the latest possible consistent state?
I thought PostgreSQL guaranteed a consistent state after a crash, am I mistaken about this?
James
James Sewell
PostgreSQL Team Lead / Solutions Architect
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On Mon, Jul 29, 2013 at 4:54 PM, Amit Langote <amitlangote09@gmail.com> wrote:
--On Mon, Jul 29, 2013 at 3:32 PM, James Sewell <james.sewell@lisasoft.com> wrote:Hey all,I understand that I have already been given an answer here, but I am still curious as to why this is the case (perhaps I should ask this on the hackers list though, if so let me know).More importantly I'd like to understand why I would need to use the start/stop backup commands to ensure a valid backup when using filesystem snapshots (assuming I get the order correct)- worst case scenario wouldn't it be the same as a crash and cause an automatic roll-forward?pg_start_backup('backup_label') and pg_stop_backup(), if I understand it correctly, write to the 'backup_label' file the information necessary to recover "consistently" from that backup. For example, backup_label file contains the checkpoint location and its REDO location (identified as "START WAL LOCATION:" field in the backup_label file.) While you are reading the code, you can read the comment above the function read_backup_label() in src/backend/access/transam/xlog.c
Amit Langote
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