Warm Standby question

Probably can. But you're talking about disabling off-host archiving.
The whole point behind this is prevention in case a host hard drive
fails... if it fails and you don't use off-host archiving then you've
lost the files you need to rebuild the database along with the original
database.

Thanks,

Scot Kreienkamp
La-Z-Boy Inc.
skreien@la-z-boy.com
734-242-1444 ext 6379
-----Original Message-----
From: pgsql-general-owner@postgresql.org
[mailto:pgsql-general-owner@postgresql.org] On Behalf Of Thomas Kellerer
Sent: Sunday, February 01, 2009 7:47 AM
To: pgsql-general@postgresql.org
Subject: [GENERAL] Warm Standby question

Hi,

(Note: I have never used log shipping before, I'm just interested in the

concepts, so I'm might be missing a very important aspect)

I was reading the blog entry about HA and warm standby:
http://scale-out-blog.blogspot.com/2009/02/simple-ha-with-postgresql-poi
nt-in-time.html

The image that explained how log shipping works, strikes me as being a
bit too
complex.
<http://1.bp.blogspot.com/_26KnjtB2MFo/SYVDrEr1HXI/AAAAAAAAAEY/ncq_AW-Vv
-w/s1600-h/pg_warm_standby.png>

According to the picture it basically works like this:

Master -> Copy master archive directory -> Copy to standby archive dir
-> copy
to pg_xlogs.

When I look at this chain I'm asking myself, why do I need the two
archive
directories?

Why can't the master copy the files directly into the pg_xlogs directory
of the
standby server?

Thanks
Thomas

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Thanks for the answer.

I see the point with the backup :)

Regards
Thomas

Scot Kreienkamp, 02.02.2009 16:19:

On Feb 1, 2009, at 4:47 AM, Thomas Kellerer wrote:

Hi,

(Note: I have never used log shipping before, I'm just interested in
the concepts, so I'm might be missing a very important aspect)

I was reading the blog entry about HA and warm standby:
http://scale-out-blog.blogspot.com/2009/02/simple-ha-with-postgresql-point-in-time.html

The image that explained how log shipping works, strikes me as being
a bit too complex.
<http://1.bp.blogspot.com/_26KnjtB2MFo/SYVDrEr1HXI/AAAAAAAAAEY/ncq_AW-Vv-w/s1600-h/pg_warm_standby.png

According to the picture it basically works like this:

Master -> Copy master archive directory -> Copy to standby archive
dir -> copy to pg_xlogs.

When I look at this chain I'm asking myself, why do I need the two
archive directories?

Why can't the master copy the files directly into the pg_xlogs
directory of the standby server?

Well, on the master the implementation has to be resilient to issues
with the archive_command and Postgres re-uses files in the pg_xlog
directory. If they weren't copied into the archive_status directory
then if a file couldn't be shipped for some reason you'd run the risk
of the file getting re-used before it was successfully pushed to the
standby.

As to where they go on the standby, remember that log shipping is not
just for warm standby implementation -- it's also a valid backup
method, i.e. archive the transaction logs along with a snapshot of the
data directory. In addition, the copy of a file into the pg_xlog
directory needs to be atomic and there's no guarantee that any given
archive_command will use a tool that does atomic copies.

Erik Jones, Database Administrator
Engine Yard
Support, Scalability, Reliability
866.518.9273 x 260
Location: US/Pacific
IRC: mage2k