RAID Capacity

Giganews Newsgroups
Subject: RAID Capacity
Posted by:  Grinder (grinder@no.spam.maam.com)
Date: Wed, 01 May 2013

I've had a couple of NAS boxes in the past, and have been fairly happy
with them.  The cost may seem a little excessive given the surplus of
Pentium 4 machines lying in the streets--maybe I should take a closer
look at FreeNAS.

At any rate, I recently purchase a 4-bay model and set it up.  An online
RAID calculator suggested I would get 8.17 TiB capacity.  In fact, the
NAS even predicted that when creating the volume in its storage manager.
  However, when it was all done, I had 8.05 TiB capacity, and (of
course) a bit less than that available.

Whence the disparity?  I asked my NAS' manufacturer, but (thusfar) they
have only given me "it's nothing to worry about" responses.  I'm not
worried, but I would still like to know where the "missing" 125 GiB has
gone.  It would be nice to have the storage, but I'm really looking to
comprehend how it works.

Can anyone enlighten me?

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You'll find my most recent response to their tech support posted below.
  It might contain salient details:

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On 4/30/2013 5:09 PM, [NAS Manufacturer's Tech Support] wrote:
>
> Hi [Grinder],
>
> As mentioned in the previous email the capacity is normal, so
> there isn't much speculation to be had, unless I'm misunderstanding
> what you're asking.
>
> Regards,
> [Tech Support Guy]

Grinder responds:

In my original posting, I went through the premises and mathematics of
why I think there should be 8.17 TiB.  Are any of those statements
incorrect?  Here they are in a more specific form.

Givens:

a) Standard RAID-5 array with 4 identical hard drives.

b) Each hard drive has 2794.5 GiB capacity.

Premises:

1) Each drive will only have 2790 GiB available capacity after the [NAS
operating system] and swap partitions are created.

2) The capacity of a RAID-5 array with 4 identical drives will be (4 -
1) x (available capacity).

Conclusion:

For these premises, my understanding is that the array's capacity will
be 3 x 2790 GiB = 8370 GiB, or 8.17 TiB.

Is that correct?  If not, what premise is bad?

This expectation, the 8.17 TiB, matches with RAID calculators, as well
as the [NAS operating system's] storage manager in its initial calculations.

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Please note that I am not asking these questions with an expectation of
finding a defect.  I want to understand why the straight-line
calculations do not match with the actual result.  Surely that extra 125
GiB has gone somewhere, and I want to understand where.

So, responding that 8.05 TiB is normal, does not really answer the
question so much as it dismisses it.

Thanks for the time you have already spent in addressing my questions.

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