[Tfug] HDD size and RAID queries

[John Gruenenfelder]

Greetings all,

It is finally time that I upgrade my file server/Mythbox machine.  It does a
whole host of other tasks, but those are the primary ones.  Currently, I have
four 500 GB SATA (3 Gb/s variety) installed, three of which are Samsung
HD502IJ drives and the fourth of which is a WDC WD5000AAKS-7.

Each drive contains two partitions, one small (~200 MB) and the other
containing the rest of the space.  The four small partitions are part of a
RAID-1 array and represent the /boot area of the system.  The other four are
part of a RAID-5 array.  That block device is then passed through LUKS and
then *that* block device goes to the LVM2 layer which finally splits it into
the various volumes.  All volumes are using XFS.

So far, this setup has worked marvelously.  Originally, I had to hack the
initrd scripts together to make it work, but now the standard Debian
initrd+kernel are capable of handling this in an entirely automated manner.

But, the time has come to upgrade.  Mostly because of MythTV, I find I am
constantly on the verge of running out of space.  I've already had to resort
to some symlink juggling to put things in different places where more space is
fre.

I have two questions that are related.  I've read a number of articles which
have presented well reasoned arguments for why RAID-5 should be abandoned in
favor of RAID-6.  I understand these arguments, yet I'm not sure if *my* big
is the same as *their* big.  That is, with my RAID-5 setup the space
efficiency is (1 - 1/n) and with four drives that works out to 2 TB * 3/4 =
1.5 TB usable space.  Clearly better than RAID-1 mirroring and I can honestly
say that, even with some of the heavy I/O that vieo streaming can do, I've
never had any issues with the RAID-5 speed (parity calculations, extra writes,
etc.)

The space efficiency for RAID-6, however, is (1 - 2/n) and with four drives
that works out to TotalSpace * 1/2 which is the same as RAID-1 mirroring.
This is the same as RAID-1 mirroring.  RAID-6 is still the better choice
because it can handle the failure of *any* two drives whereas if I were to use
RAID-1 I would have to create two mirrors of two drives each and such a setup
could only handle two drive failures in particular circumstances.

I'm looking at buying four new drives, either 2 TB or 3 TB in size.  Part of
that decision will depend on what type of RAID I use.  RAID-5 with four 2 TB
drives gives me 6 TB usable space, but only 4 TB if I choose RAID-6.  Using 3
TB drives, the numbers are 9 TB and 6 TB for RAID-5 and RAID-6, respectively.

The argument to ditch RAID-5 in favor of RAID-6 is entirely based on
probabilities and the likelyhood of encountering an unrecoverable error while
rebuilding an array.  Am I actually in this realm with these drive sizes?  Is
my Big Array even remotely close to the Big Array these authors are concerned
with?  If it's not, then I can stay with RAID-5 and gain an extra 25% usable
disk space.  Confusing...


The other related question deals with using these new giant drives to begin
with.  I came across a few vague sentences that *seemed* to indicate that the
current BIOS methods for accessing large drives (LBA) won't do the trick for
these 1 TB+ drives and that UEFI is required.  Is this really true?  The
motherboard they will be used with is not new, nor is it very old either.
I've used 1 TB+ drives on a much older machine at work, but they were accessed
via USB2 and FireWire and not via a SATA or eSATA port.  Since I saw this
mentioned in only one place I tend to think that this isn't true, but I
thought I should check first.



-- 
--John Gruenenfelder    Systems Manager, MKS Imaging Technology, LLC.
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