Message-ID: <4B74118C.30704@enterprisedb.com>
Date: Thu, 11 Feb 2010 16:17:48 +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: <20100127152751.3B2047541B9@cvs.postgresql.org>
	<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>
In-Reply-To: <20100211140118.GB14128@oak.highrise.ca>
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Aidan Van Dyk wrote:
> But colour me confused, I'm still not understanding why this is any
> different that with normal PITR recovery.
> 
> So even with a plain "cp" in your recovery command instead of a
> sleep+copy (a la pg_standby, or PITR tools, or all the home-grown
> solutions out thery), I'm not seeing how it's going to get "half files".

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.

> It's well know in PostgreSQL wal archivne - you don't just "shove" files
> into the archive, you make sure they appear there with the right name
> atomically.  And if the master is only running the archive command on
> whole WAL files, I just don't understand this whole short wal problem.

Yeah, if you're careful about that, then this change isn't required. But
pg_standby protects against that, so I think it'd be reasonable to have
the same level of protection built-in. It's not a lot of code.

We could well just document that you should do that, ie. make sure the
file appears in the archive atomically with the right size.

> And don't try and tell me your just "poaching" files from a running
> cluster's pg_xlog directory, because I'm going to cry...

No :-).

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