Date: Wed, 2 Apr 2008 19:49:58 -0400 (EDT)
From: Greg Smith <gsmith@gregsmith.com>
To: James Mansion <james@mansionfamily.plus.com>
cc: pgsql-performance@postgresql.org
Subject: Re: POSIX file updates
In-Reply-To: <47F40D54.2020700@mansionfamily.plus.com>
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On Wed, 2 Apr 2008, James Mansion wrote:

> I'm well aware that there are battery-backed caches that can be detached 
> from controllers and moved.  But you'd better make darn sure you move 
> all the drives and plug them in in exactly the right order and make sure 
> they all spin up OK with the replaced cache, because its expecting them 
> to be exactly as they were last time they were on the bus.

The better controllers tag the drives with a unique ID number so they can 
route pending writes correctly even after such a disaster.  This falls 
into the category of tests people should do more often but don't:  write 
something into the cache, pull the power, rearrange the drives, and see if 
everything still recovers.

> You would think hard drives could have enough capacitor store to dump 
> cache to flash or the drive - if only to a special dump zone near where 
> the heads park. They are spinning already after all.

The free market seems to have established that the preferred design model 
for hard drives is that they be cheap and fast rather than focused on 
reliability.  I rather doubt the tiny percentage of the world who cares as 
much about disk write integrity as database professionals do can possibly 
make a big enough market to bother increasing the cost and design 
complexity of the drive to do this.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD