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Re: 8192 BLCKSZ ?

From: Matthew Kirkwood <matthew(at)hairy(dot)beasts(dot)org>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: 8192 BLCKSZ ?
Date: 2000-11-29 13:09:05
Message-ID: Pine.LNX.4.10.10011291306070.19733-100000@sphinx.mythic-beasts.com (view raw or flat)
Thread:
Lists: pgsql-hackers
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.

Matthew.


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