Greetings, * julyanto SUTANDANG (julyanto@xxxxxxxxxxxx) wrote: > Please elaborate more of what you are saying. What i am saying is based on > the Official Docs, Forum and our own test. This is what we had to do to > save time, both backing up and restoring. > > https://www.postgresql.org/docs/9.6/static/functions-admin.html > > When PostgreSQL in the mode of Start Backup, PostgreSQL only writes to the > XLOG, then you can safely rsync / copy the base data (snapshot) then later > you can have full copy of snapshot backup data. You are confusing two things. After calling pg_start_backup, you can safely copy the contents of the data directory, that is correct. However, PostgreSQL *will* continue to write to the data directory. That, however, is ok, because those changes will *also* be written into the WAL and, after calling pg_start_backup(), you collect all of the WAL using archive_command or pg_receivexlog. With all of the WAL which was created during the backup, PG will be able to recover from the changes made during the backup to the data directory, but you *must* have all of that WAL, or the backup will be inconsistent because of those changes that were made to the data directory after pg_start_backup() was called. In other words, if you aren't using pg_receivexlog or archive_command, your backups are invalid. > if you wanted to backup in later day, you can use rsync then it will copy > faster because rsync only copy the difference, rather than copy all the > data. This is *also* incorrect. rsync, by itself, is *not* safe to use for doing that kind of incremental backup, unless you enable checksums. The reason for this is that rsync has only a 1-second level granularity and it is possible (unlikely, though it has been demonstrated) to miss changes made to a file within that 1-second window. > my latter explanation is: use pg_basebackup, it will do it automatically > for you. Yes, if you are unsure about how to perform a safe backup properly, using pg_basebackup or one of the existing backup tools is, by far, the best approach. Attempting to roll your own backup system based on rsync is not something I am comfortable recommending any more because it is *not* simple to do correctly. Thanks! Stephen
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