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Date: Mon, 11 Apr 2011 10:47:12 -0500
From: "Kevin Grittner" <Kevin.Grittner@wicourts.gov>
To: "Ian Lea" <ian.lea@gmail.com>, "David Roland" <david.roland@soapware.com>
Cc: <pgsql-admin@postgresql.org>
Subject: Re: pg_start_backup - backups
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"David Roland" <david.roland@soapware.com> wrote:
 
> changes to the data folder continue even after the PG_START_BACKUP
> command has been issued. This implies to me that the contents of
> any copy of the data folder may be unreliable. i.e. the copy may
> not reflect the state of the data folder either before the copy
> started or after the copy has finished. It may reflect the state
> of the data folder in some transient form.
 
Right.
 
> Assuming this is true, is the copy still usable for restoration?
 
Yes.
 
> If so, how does PostgreSQL get the data folder to a stable
> state? Is it by the use of the WAL files that may be created
> during the backup process and the restore.config file?
 
Exactly.  Simplifying somewhat:
 
- The pg_start_backup causes the WAL position to be remembered.  You
  will need to start WAL replay at this point.
 
- Every significant change to a page is WAL-logged.
 
- Changes after you record the restart point may or may not be in
  the base backup.
 
- After the base backup copying is complete, pg_stop_backup records
  the WAL position.  You will need to replay WAL *at least* to this
  point to have a consistent database.
 
- WAL replay will bring every page modified after the "start" to a
  valid state, whether or not any or all direct modifications to
  that page made it into the base backup.
 
- This actually goes beyond page fix-ups to file creation and
  deletion, etc.
 
-Kevin