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Re: Cluster feature: Start/stop archiving at runtime

From: Tatsuo Ishii <ishii(at)sraoss(dot)co(dot)jp>
To: masao(dot)fujii(at)gmail(dot)com
Cc: martin(dot)pihlak(at)gmail(dot)com, greg(at)2ndquadrant(dot)com, pgsql-cluster-hackers(at)postgresql(dot)org
Subject: Re: Cluster feature: Start/stop archiving at runtime
Date: 2010-03-02 07:42:09
Message-ID: 20100302.164209.106827636.t-ishii@sraoss.co.jp (view raw or flat)
Thread:
Lists: pgsql-cluster-hackers
> On Sun, Feb 28, 2010 at 9:08 PM, Martin Pihlak <martin(dot)pihlak(at)gmail(dot)com> wrote:
> > Greg Smith wrote:
> >> Next up is "Start/stop archiving at runtime".  First set of questions:
> >>
> >> -Is this referring to the current PITR archiving as done via
> >> archive_command, does it intend to cover a wider scope than that, or is
> >> "archiving" being used in a general replication sense instead not
> >> related to that?
> >>
> >> -What is the issue with Skype and failed nodes this is alluding to?
> >>
> >
> > AFAIK the only issue is that we cannot set archive_mode without restarting
> > Postgres. This leads to archive_mode always set to "on" and a dummy
> > archive_command being used. The background for this is that we don't know
> > in advance when we need to create a PITR clone of a particular database.
> 
> Changing archive_command and reloading the postgresql.conf (signaling
> a SIGHUP) is not sufficient for you? You only need more user-friendly
> API to do that? Or more intelligent functionality?

IMO it's not sufficient. According to the docs:

"If archive_command is an empty string (the default) while
archive_mode is enabled, WAL archiving is temporarily disabled, but
the server continues to accumulate WAL segment files in the
expectation that a command will soon be provided.  Setting
archive_mode to a command that does nothing but return true,
e.g. /bin/true, effectively disables archiving, but also breaks the
chain of WAL files needed for archive recovery, so it should only be
used in unusual circumstances"

My interpretation of this paragraph is, if we set archive_command to
empty string, PostgreSQL accumulates all WAL under pg_xlog. i.e. no
WAL resuing occurs. Maybe set archive_command to /bin/true is enough
for pgpool-II, but it needs to rewrite postgresql.conf by pgpool. That
may make users to feel uncomfortable, I'm afraid.
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese: http://www.sraoss.co.jp

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