[mythtv-users] Reassemble RAID with mdadm

[paul10 at planar.id.au]

On RAID6, RAID5, RAID0+1 and others.

On a modern CPU, there is a theoretical slowdown calculating parity, but
my quad core runs about 5% of one core when I'm thrashing the drives hard. 
Frankly, any modern system has CPU to burn, and I don't believe parity
calcs are a real-life impact on performance.

RAID5 is the most space/cost efficient.  You get n-1 space, so with a 5
drive array, you get 4 disks worth of space.

I had a lot of problems with RAID5 and consumer grade disks.  My disks are
generally failing about 1 every 3-6 months, in a 6 disk array.  Which makes
sense if you figure the life of a drive is 2-3 years.  My theory here is
that consumer grade drives are made to a budget.  They give a 3 year
warranty, but they're assuming you don't run it 24x7. My myth setup runs
24x7 and is usually reading or writing - which means the drives never
really idle out.  I think someone (in all the various drive manufacturers)
has figured it is cheaper to replace drives under warranty for the handful
of mythtv users who run 24x7 than it is to build a good drive for the 99%
of users who turn their computer on once a day.

My problem is that if I lose a drive once every 3-6 months, and it takes
me a couple of weeks to get around to a replacement, then there is a real
chance of losing another disk in that timeframe.  Actually, if you're a
klutz like me, more likely is you'll lose one drive, decide that, really,
my machine is hot pluggable (it is, honest), pull out the dead drive to put
in a new one, and accidentally knock out the cable for one of the other
drives.  Broken array.  

I thought about going RAID5 with a spare, but my understanding was that if
you lose a drive, it immediately goes into rebuild. Which entails reading
every sector on every drive, and that can trigger another drive to fail -
result is broken array again.  With RAID6, when one drive fails what you
have left is essentially RAID5.  When you put a new drive in, it rebuilds,
but the odds of two drives failing during that rebuild are lower - so
safer.

Anyway, long story short, I'm running RAID6 with 6x1TB, giving 4TB
useable.  With ext4, the performance is sufficient that I'm more than
capable of servicing 3xtuners at HD quality, plus commercial flagging, plus
playback on 2 frontends.  In theory other configurations might be faster,
but this is fast enough, low maintenance, relatively cheap, and easy to
configure.