Message-ID: <4B7438A9.8090902@enterprisedb.com>
Date: Thu, 11 Feb 2010 19:04:41 +0200
From: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
Organization: EnterpriseDB
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To: Aidan Van Dyk <aidan@highrise.ca>
CC: Simon Riggs <simon@2ndQuadrant.com>, Fujii Masao <masao.fujii@gmail.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Re: [COMMITTERS] pgsql: Make standby server	continuously
	retry  restoring the next WAL
References: <3f0b79eb1002092105r21e009d3v468496058ba04392@mail.gmail.com>
	<4B726120.80007@enterprisedb.com>
	<1265884657.7341.1192.camel@ebony>
	<4B73F678.8070109@enterprisedb.com>
	<1265891248.7341.1346.camel@ebony>
	<4B73FB99.4080403@enterprisedb.com>
	<1265893599.7341.1454.camel@ebony>
	<4B740613.5090004@enterprisedb.com>
	<20100211140118.GB14128@oak.highrise.ca>
	<4B74118C.30704@enterprisedb.com>
	<20100211144204.GC14128@oak.highrise.ca>
In-Reply-To: <20100211144204.GC14128@oak.highrise.ca>
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Aidan Van Dyk wrote:
> * Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> [100211 09:17]:
> 
>> If the file is just being copied to the archive when restore_command
>> ('cp', say) is launched, it will copy a half file. That's not a problem
>> for PITR, because PITR will end at the end of valid WAL anyway, but
>> returning a half WAL file in standby mode is a problem.
> 
> But it can be a problem - without the last WAL (or at least enough of
> it) the master switched and archived, you have no guarantee of having
> being consistent again (I'm thinking specifically of recovering from a
> fresh backup)

You have to wait for the last WAL file required by the backup to be
archived before starting recovery. Otherwise there's no guarantee anyway.

>> We could well just document that you should do that, ie. make sure the
>> file appears in the archive atomically with the right size.
> 
> I have to admit, today was the first time I went and re-read the PITR
> docs, and no, the docs don't seem to talk about that... Maybe it was
> just plain obvious to me because it (the atomic apperance) is something
> unix devloppers have always had to deal with, so it's ingrained in me.
> But I'm *sure* that I've seen that bandied around as common knowledge on
> the lists, and one of the reasons we alway see warnings about using
> rsync instead of plain SCP, etc.
> 
> So ya, we should probably mention that somewhere in the docs.  Section
> 24.3.6. Caveats?

-1. it isn't necessary for PITR. It's a new requirement for
standby_mode='on', unless we add the file size check into the backend. I
think we should add the file size check to the backend instead and save
admins the headache.

-- 
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com