Our documentation suggests that you can take a base backup of a warm
standby server while it's running:

> If we take a backup of the standby server's data directory while it is 
> processing logs shipped from the primary, we will be able to reload that data 
> and restart the standby's recovery process from the last restart point. We no 
> longer need to keep WAL files from before the restart point. If we need to 
> recover, it will be faster to recover from the incrementally updated backup 
> than from the original base backup. 

That doesn't seem safe. If the server makes a new restartpoint while the
backup is running, and pg_control is backed up after the new
restartpoint is made, recovery will restart from the new restartpoint.
That is wrong; recovery needs to restart at the restartpoint that was
most recent when the backup started. This is basically the same issue we
have solved in master with the backup_label file.

I wonder if it would be enough to document that pg_control must be
backed up first?

  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com

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