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Re: beta testing version

From: Don Baccus <dhogaza(at)pacifier(dot)com>
To: pgsql-hackers(at)postgresql(dot)org, pgsql-hackers(at)postgresql(dot)org
Subject: Re: beta testing version
Date: 2000-12-01 05:39:14
Message-ID: 3.0.1.32.20001130213914.01799d70@mail.pacifier.com (view raw or flat)
Thread:
Lists: pgsql-hackers
At 03:35 PM 11/30/00 -0800, 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.  

Yeah, I posted similar stuff to the PHPbuilder forum in regard to
PG.

>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.  

Let's put it this way ... Oracle, a transaction-safe DB with REDO
logging, has for a very long time implemented disk mirroring.  Now,
why would they do that if you could pull the plug on the processor
and depend on REDO logging to save you?

And even then you're expected to provide adequate power backup to
enable clean shutdown.

The real safety you get is that your battery sez "we need to shut
down!" but has enough power to let you.  Transactions in progress
aren't logged, but everything else can tank cleanly, and your DB is
in a consistent state.  

Mirroring protects you against (some) disk drive failures (but not
those that are transparent to the RAID controller/driver - if your
drive writes crap to the primary side of the mirror and no errors
are returned to the hardware/driver, the other side of the mirror
can faithfully reproduce them on the mirror!)

But since drives contain bearings and such that are much more likely
to fail than electronics (good electronics and good designs, at least),
mechanical failure's more likely and will be known to whatever is driving
the drive.  And you're OK then...



- Don Baccus, Portland OR <dhogaza(at)pacifier(dot)com>
  Nature photos, on-line guides, Pacific Northwest
  Rare Bird Alert Service and other goodies at
  http://donb.photo.net.

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