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Re: page corruption on 8.3+ that makes it to standby

From: Jeff Davis <pgsql(at)j-davis(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: page corruption on 8.3+ that makes it to standby
Date: 2010-07-27 22:23:04
Message-ID: 1280269384.8505.118.camel@jdavis-ux.asterdata.local (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, 2010-07-27 at 17:18 -0400, Robert Haas wrote:
> > My first concern with that idea was that it may create an inconsistency
> > between the primary and the standby. The primary could have a bunch of
> > zero pages that never make it to the standby.
> 
> Maybe I'm slow on the uptake here, but don't the pages start out
> all-zeroes on the standby just as they do on the primary? The only way
> it seems like this would be a problem is if a page that previously
> contained data on the primary was subsequently zeroed without writing
> a WAL record - or am I confused?

The case I was concerned about is when you have a table on the primary
with a bunch of zero pages at the end. Then you SET TABLESPACE, and none
of the copied pages (or even the fact that they exist) would be sent to
the standby, but they would exist on the primary. And later pages may
have data, so the standby may see page N but not N-1.

Generally, most of the code is not expecting to read or write past the
end of the file, unless it's doing an extension.

However, I think everything is fine during recovery, because it looks
like it's designed to create zero pages as needed. So your idea seems
safe to me, although I do still have some doubts because of my lack of
knowledge in this area; particularly hot standby conflict
detection/resolution.

My idea was different: still log the zero page, just don't set LSN or
TLI for a zero page in log_newpage() or heap_xlog_newpage(). This isn't
as clean as your idea, but I'm a little more confident that it is
correct.

Regards,
	Jeff Davis



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