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Incrementally Updated Backups: Docs Clarification

From: "Thomas F(dot) O'Connell" <tf(at)o(dot)ptimized(dot)com>
To: pgsql-general(at)postgresql(dot)org
Subject: Incrementally Updated Backups: Docs Clarification
Date: 2007-04-19 20:48:34
Message-ID: 09318B6D-8C60-4287-978E-EAE199ACA15E@o.ptimized.com (view raw or flat)
Thread:
Lists: pgsql-docspgsql-general
I'll preemptively apologize for cross-posting this, but I follow the  
lists via Google Groups, and I never saw this show up on -general  
there. I see it in the archives, but the lack of discussion on - 
general makes me wonder if it was widely read. Since it involves what  
I perceive to be a lack of clarity in the docs, I'll offer to make up  
for my cross-posting by helping to improve the docs as mentioned  
below once I get things working...

I'm about to begin playing with incrementally updated backups for a  
warm standby scenario, but I need some help understanding this  
paragraph in postgres terms. From 23.4.5 in the 8.2.3 docs:

"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?

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

"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?

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? 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.

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

Is that all there is to it?

--
Thomas F. O'Connell

optimizing modern web applications
: for search engines, for usability, and for performance :

http://o.ptimized.com/
615-260-0005

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