Re: POSIX file updates

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(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD