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Re: Online base backup from the hot-standby

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Cc: Steve Singer <ssinger_pg(at)sympatico(dot)ca>, Jun Ishiduka <ishizuka(dot)jun(at)po(dot)ntts(dot)co(dot)jp>, magnus(at)hagander(dot)net, heikki(dot)linnakangas(at)enterprisedb(dot)com, pgsql-hackers(at)postgresql(dot)org, robertmhaas(at)gmail(dot)com, cedric(dot)villemain(dot)debian(at)gmail(dot)com
Subject: Re: Online base backup from the hot-standby
Date: 2012-01-24 10:54:56
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Lists: pgsql-hackers
On Tue, Jan 24, 2012 at 9:51 AM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:

>> I'll proceed to commit for this now.
> Thanks a lot!

Can I just check a few things?

You say
+        * Update full_page_writes in shared memory and write an
+        * XLOG_FPW_CHANGE record before resource manager writes cleanup
+        * WAL records or checkpoint record is written.
+        */

why does it need to be before the cleanup and checkpoint?

You say
+        * Currently only non-exclusive backup can be taken during recovery.
+        */


You mention in the docs
"The backup history file is not created in the database cluster backed up."
but we need to explain the bad effect, if any.

You say
"If the standby is promoted to the master during online backup, the
backup fails."
but no explanation of why?

I could work those things out, but I don't want to have to, plus we
may disagree if I did.

There are some good explanations in comments of other things, just not
everywhere needed.

What happens if we shutdown the WALwriter and then issue SIGHUP?

Are we sure we want to make the change of file format mandatory? That
means earlier versions of clients such as pg_basebackup will fail
against this version. Should we allow that if BACKUP FROM is missing
we assume it was master?

There are no docs to explain the new feature is available in the main
docs, or to explain the restrictions.
I expect you will add that later after commit.

 Simon Riggs         
 PostgreSQL Development, 24x7 Support, Training & Services

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