Tuesday, October 24

What does pg_start_backup() do?

Reading mailing lists can damage your health, as I recently discovered on the PostgreSQL Performance list where backup was being discussed.

First off, don’t read blogs for finding out critical pieces of info. Read the docs because they are accurate, fully reviewed and well maintained.

I should add that I was the initial author of them as well, so maybe it’s OK to carry on reading…

pg_start_backup() is a function we execute to start a base backup. It was part of the original API for physical backup introduced in PostgreSQL 8.0. It’s now been mostly superceded by the replication command BASE_BACKUP, which is most frequently executed by the pg_basebackup utility.

So what does a base backup actually do? Well, first we execute a checkpoint so that as many changed data blocks are on disk as possible. Next we force full page writes to occur, even if full_page_writes = off, because we need to see the whole page for any changes. Lastly, we record the starting point of the backup. That’s all.

Base backup does NOT prevent writes to the data directory. It’s designed to be “fully online” so it doesn’t take locks on objects, doesn’t interefere with the operation of the database apart from some details if you try to shut it down while taking a backup.

pg_stop_backup() is the end marker for that backup.

The key point is that the base backup is NOT a consistent copy of the database. You might have copied every file, but all the data is taken at different times. So its wrong. Until you recover the database with the WAL changes that occurred between the start backup and the stop backup.

Which is why you’ll be wanting to use a command like this

pg_basebackup –xlog-method=stream

or use a utility that does everything for you, like Barman.

2 Comments

  • Adam Scott

    Great reminder on pg_start_backup()!

    I’ve never tried a restore without the WAL files. I’m guessing there would be a complaint of missing WAL files. Looking through xlog.c (line 7196), I’m guessing you will see a message along the lines of: “WAL ends before end of online backup”.

    So when one performs their scheduled test recovery and you see that message, you know you aren’t getting consistent backups.

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