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Subject: Re: [DOCS] Incrementally Updated Backups: Docs Clarification
From: "Simon Riggs" <simon@2ndquadrant.com>
To: "Thomas F. O'Connell" <tf@o.ptimized.com>
Cc: <pgsql-general@postgresql.org>
In-Reply-To: <09318B6D-8C60-4287-978E-EAE199ACA15E@o.ptimized.com>
References: <09318B6D-8C60-4287-978E-EAE199ACA15E@o.ptimized.com>
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Date: Wed, 25 Apr 2007 15:42:33 +0100
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On Thu, 2007-04-19 at 15:48 -0500, Thomas F. O'Connell wrote:

> "If we take a backup of the standby server's files while it is
> following 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."
> 
> 
> I'm specifically confused about the meaning of the following phrases:
> 
> 
> "backup of the standby server's files" - Which files?

the files that make up the database server:
- data directory
- all tablespace directories

> "reload that data" - What does this mean in postgres terms?

copy back from wherever you put them in the first place

"that data" referring to the "files that make up the db server"

> "last restart point" - What is this? Wouldn't it be able to restart
> from the last recovered file, which would presumably occur later than
> the last restart point?

No, we don't restart file-by-file. 

http://developer.postgresql.org/pgdocs/postgres/continuous-archiving.html#BACKUP-PITR-RECOVERY

"If recovery finds a corruption in the WAL..." onwards explains the
restart mechanism. It's much like checkpointing, so we don't restart
from the last log file we restart from a point possibly many log files
in the past.

> Does this mean make a filesystem backup of the standby server's data
> directory while it's stopped, and then start it again with that data
> and the restricted set of WAL files needed to continue recovery? 

No need to stop server. Where do you read you need to do that?

> I'd like to see the language here converted to words that have more
> meaning in the context of postgres. I'd be happy to attempt a revision
> of this section once I'm able to complete an incrementally updated
> backup successfully.

Feel free to provide updates that make it clearer.

> Here's how I envision it playing out in practice:
> 
> 
> 1. stop standby postgres server
> 2. [optional] preserve data directory, remove unnecessary WAL files
> 3. restart standby server

step 2 only. 

Clearly not an optional step, since its a 1 stage process. :-)

-- 
  Simon Riggs             
  EnterpriseDB   http://www.enterprisedb.com