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Re: 8192 BLCKSZ ?

From: mlw <markw(at)mohawksoft(dot)com>
To: Matthew Kirkwood <matthew(at)hairy(dot)beasts(dot)org>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: 8192 BLCKSZ ?
Date: 2000-11-29 13:26:16
Message-ID: 3A2503F8.95F21FB@mohawksoft.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Matthew Kirkwood wrote:
> 
> On Tue, 28 Nov 2000, Tom Lane wrote:
> 
> > Nathan Myers <ncm(at)zembu(dot)com> writes:
> > > In the event of a power outage, the drive will stop writing in
> > > mid-sector.
> >
> > Really?  Any competent drive firmware designer would've made sure that
> > can't happen.  The drive has to detect power loss well before it
> > actually loses control of its actuators, because it's got to move the
> > heads to the safe landing zone.  If it checks for power loss and
> > starts that shutdown process between sector writes, never in the
> > middle of one, voila: atomic writes.
> 
> In principle, that is correct.  However, the SGI XFS people
> have apparently found otherwise -- what can happen is that
> the drive itself has enough power to complete a write, but
> that the disk/controller buffers lose power and so you end
> up writing a (perhaps partial) block of zeroes.

I have worked on a few systems that intend to take a hard power failure
gracefully. It is a very hard thing to do, with a lot of specialized
circuitry.

While it is nice to think about, on a normal computer systems one can
not depend on a system shutting down gracefully on a hard power loss
without a smart UPS and daemon to shut down the system.

It does not matter one bit about disk write sizes or what ever. Unless
the computer can know it is about to lose power, it can not halt its
operations and enter a safe mode.

The whole "pull the plug" mentality is silly. Unless a system hardware
is specifically designed to manage this and proper software in place, it
can not be done, and any "compliance" you think you see is simply luck.

Any computer that has important data should have a smart UPS and a
daemon to manage it. 

-- 
http://www.mohawksoft.com

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