[mythtvnz] Myth & Raid. SW or HW..

[Steve Hodge]

On Wed, Jul 18, 2012 at 8:42 PM, Johan Schuld <johanschuld at gmail.com> wrote:

> So, as HD will inevitably die, my plan is to buy some new HD's and put
> them in a RAID array. Thinking RAID1 as I currently don't have the funds to
> buy more than 2 drives.


I assume your goal is data security? RAID is not a substitute for backups
so if you have really critical data on that machine you should be backing
it up.


> I've done quite some research so far about the options and wondered what
> the easiest and most reliable solution. I.e. I know people having a HW Raid
> using a Dell Perc 5/i card. These are so common they can be picked up for
> less than $100NZ inc shipping from ebay and with some SAS to SATA cables
> they run fine with SATA drives.  A cheaper option would obviously be SW
> Raid, but not that much cheaper.
>

$100 is about 60% of the price of a 2TB drive. So you can have 2TB of
storage on hardware RAID 1 for about $420 or you can have 4TB of storage on
software RAID 5 for $480. There are still a lot of people who favour
hardware RAID but there really aren't many advantages unless you go high
end. The CPU hit for software RAID is very low for modern CPUs and software
RAID is far more flexible. Keep in mind that hardware RAID involves an
extra device so it actually increases the chance of failures. And if the
controller fails and you're running something like RAID 5 or 6 you might
not be able to recover the array without replacing the controller card with
an identical model. That could be difficult 4 or 5 years down the track.


> As Raid1 is really not that much overhead, should I just not bother with
> HW raid, and let linux do the job? (Opensuse 12.1) . Can I transfer a
> already existing non-raid install to a SW raid1 config? For now I'll run
> the OS and media storage on one raid volume, I might split that out later
> when I can afford some more drives.


Yes, with mdadm you can set up a degraded array. You can also mirror your
existing drive. So you could put in one new 2TB drive and mirror the
existing drive to the new one. Keep them in RAID1 until the old drive
fails. Use the remaining space on the new drive as separate storage group
in Myth. Later you can add another 2TB drive and mirror both the OS part
and the storage group if you like (or you could use the extra space on the
new drive as a 2nd storage group). If you can get 2 drives now you could
run them as a degraded RAID 5, but note that a degraded RAID 5 is
effectively just a RAID 0 (if either drive fails you lose everything). I
wouldn't run that way for any length of time.

My recommendation would be three 2TB drives in software RAID 5. If you
really can't stretch to 3 drives then look at mirroring the existing 250GB
across 2 drives and use the remaining space as separate storage groups.
That'll give you 250GB of mirrored capacity and 3.5TB additional space.
That way you maximise capacity and yet still have a fair bit of protection
from data loss - you might still lose half your recordings but if go with
RAID1 you can only record half as much anyway.

Personally I don't bother with LVM. mdadm can be used to reshape RAID
arrays so you can increase capacity by adding disks later and then simply
use jfs or xfs to extend the filesystem to take advantage of the larger
size. I've done this myself in moving from a 3x1.5TB RAID 5 to a 4x1.5TB
RAID 5.

Cheers,
Steve
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ourshack.com/pipermail/mythtvnz/attachments/20120718/4fe79be0/attachment-0001.html>