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Re: beta testing version

From: The Hermit Hacker <scrappy(at)hub(dot)org>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: beta testing version
Date: 2000-12-01 04:00:12
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Thu, 30 Nov 2000, Nathan Myers wrote:

> On Thu, Nov 30, 2000 at 07:02:01PM -0400, The Hermit Hacker wrote:
> > 
> > v7.1 should improve crash recovery ...
> > ... with the WAL stuff that Vadim is producing, you'll be able to
> > recover up until the point that the power cable was pulled out of 
> > the wall.
> Please do not propagate falsehoods like the above.  It creates
> unsatisfiable expectations, and leads people to fail to take
> proper precautions and recovery procedures.  
> After a power outage on an active database, you may have corruption
> at low levels of the system, and unless you have enormous redundancy
> (and actually use it to verify everything) the corruption may go 
> undetected and result in (subtly) wrong answers at any future time.
> The logging in 7.1 protects transactions against many sources of 
> database crash, but not necessarily against OS crash, and certainly
> not against power failure.  (You might get lucky, or you might just 
> think you were lucky.)  This is the same as for most databases; an
> embedded database that talks directly to the hardware might be able
> to do better.  

We're talking about transaction logging here ... nothing gets written to
it until completed ... if I take a "known to be clean" backup from the
night before, restore that and then run through the transaction logs, my
data should be clean, unless my tape itself is corrupt.  If the power goes
off half way through a write to the log, then that transaction wouldn't be
marked as completed and won't roll into the restore ...

if a disk goes corrupt, I'd expect that the redo log would possibly have a
problem with corruption .. but if I pull the plug, unless I've somehow
damaged the disk, I would expect my redo log to be clean *and*, unless
Vadim totally messed something up, if there is any corruption in the redo
log, I'd expect that restoring from it would generate from red flags ...

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