[mythtv-users] Reassemble RAID with mdadm

[MarcT]

> Believe you can do a raid5 configuration with 3 drives.  However
> generating the parity bit slows things down on the write.
> 
> With 4 drives you would be better using RAID 0+1

RAID 5 requires a mimimum of 3 yes, but at that point it's not significantly
different than a RAID 3 setup.

IMO RAID 0+1 is the dumbest use of available HDD's. Yes you are fault
tolerant but if you lose 1 drive from each stripe you are hosed. RAID 10
would be a little better as you can lose 1 drive from each mirror but if you
lose both from the same mirror you are in the same boat.

I have not seen a performance hit in my system running RAID 5. You get some
of the benefits of a stripe with the fault tolerance of being able to lose 1
drive, you also don't lose the storage from the extra drive. For example, in
a RAID 10 or 0+1 you are using 4 drives but your total storage is no more
than having 2 drives striped, with fault tolerance. RAID 5 gives you the
storage space of all 4 drive minus the space needed for parity, essentially
4-1, so your total storage space is about that of a 3 drive stripe with the
ability to lose one drive and not lose the data.

RAID 6 would be the best setup (4-2, similar to the 4 drive 10 or 0+1 setup
but you can lose any 2 drives ant not 1 drive from each mirror/stripe)
However, for software RAID, I don't think we are ready for that yet with
today's systems and you would see the performance hit that was commented on
above. You would need a hardware RAID controller to make use of RAID 6
without seeing a significant hit to performance.

My RAID 5 drive is only used for media storage, I have a raid mirror setup
for the / and /boot partitions.
As of this time I have been able to capture 6 concurrent HD recordings, 2
per HD tuner, without incident.

MarcT