On Friday 20 April 2007 20:52, Nick Holland wrote:
> Well, now a year and a half later, we are over
> 9GB/day, so our original math was dead wrong, so we would have
> managed to have bought both TOO MUCH and TOO LITTLE storage up
> front, at the same time, and paid way too much...and half way
> through the project's archive life, the storage would look
> pathetically low tech.
As Nick tried to point out, buying all of the storage you'll eventually
need for a project "right now" generally proves to be a waste of money
over the long haul. Adding storage incrementally is normally cheaper.
There is one, single, very sad exception to the above rule:
Poorly Configured Hardware RAID sets.
I've seen this problem a lot. Somebody decides they want to put "X"
disks in a RAID-5 array, so they buy a hardware controller and the "X"
disks, then configure the whole thing according to the controller
directions... -Dumb Move.
Since you followed along with your controller documentation to set up
your RAID-5 across all the disks, you probably didn't notice that you
used all of the space on each of the drives -- or you didn't think
about the ramifications of using all of the space.
Guess what? -If one of those disks fail, you may be stuffed. The reason
is simply that you may not be able to get another disk of the exact
same size -- EVEN IF IT HAS AN IDENTICAL PART NUMBER.
Most people do not realize there are subtle differences between disks,
yes, even between disks with the exact same part number. When
manufactured, one batch of drives might allow for slightly more or less
space than the next batch of drives manufactured. As long as the drives
will hold at least "N" number of bits and they satisfy the marketing or
packaging claims (i.e. "500 Gigbytes" or whatever), they get sold.
So all of your original disks looked identical and allowed you to use
the same exact size for the total space on each. And since you followed
the directions, you used all of it. The trouble is finding a correct
replacement when one of the originals fail.
If you are really lucky, then you might get a replacement disk that is
just slightly larger than the ones you originally purchased. In this
case you waste a few megabytes when you configure it to only use the
same size as the others in the RAID. The same is sometimes true if you
can buy a replacement disk with greater capacity than the originals.
In far too many cases, a supposedly same size replacement disk is just a
few megabytes smaller than your originals, so it is completely useless
as a replacement. You might go through this process of buying a
replacement and finding out it's useless a few times before finally
getting lucky. On the other hand, you might not ever get lucky.
Equally, though it's generally bad juju to go mixing disks from
different vendors in a RAID, if a vendor goes out of business, you
sometimes don't get a choice about it.
Generally, the best thing you can do to avoid this problem is to be
smart enough to only use 95% (or so) of the available space on each
disk when configuring your RAID.
That 5% (or so) of wasted space is your insurance policy for *when* a
disk fails, you'll have a much better chance at getting a working
replacement.
Of course, some wise ass is going to say buy "X" extra disks to keep
around as hot/cold spares. Yes, it's not a bad idea if you have the
money to burn, but what happens when you run out of spares? -Yep,
you're still stuffed and have to go through the problem of finding a
replacement which will work. I'll take a good insurance policy over a
stack of (possibly non-working) spares any day.
So the "exception" to the "buy your storage incrementally" rule only
occurs if you absolutely insist on using the entire size of the disk
when configuring your raid. If you've done that, then you must have an
identical (and tested) spare available. And yes, Murphy's Law states
you'll accidentally drop the spare while on your way to the server room
to replace a failed disk.
kind regards,
jcr
