[mythtv-users] anyone using software RAID 5 for 1 terabyte +

Richard Freeman r-mythtv at thefreemanclan.net
Sun Aug 6 21:59:26 UTC 2006

Marco Nelissen wrote:
>> The only bad thing about the raid 5 is that it cant be added to.  Not 
>> sure how to address that one.
> Have a look at evms.

As far as I can tell that doesn't support anything along these lines
that can't be done with md and lvm.  If I have 5x250GB drives in a 1TB
RAID-5 with one redundant drive, and I want to add a single 250GB drive
to the array to obtain 1.25TB of storage, this is not trivial to
accomplish with any of the tools (actually, this can be done with md
offline at some risk in the event of a power failure).

What I would like to see is a solution for mixtures of drive sizes.
Suppose I have 4 120GB drives in RAID-5 and I want to add a 250GB drive
and a 60GB drive to the array.  In theory this can be accomplished by
chopping everything into 10GB blocks and having 25-block redundancy (so
that the array could handle the loss of the 250GB drive, or two of the
120GB drives).  Then if I add a 500GB drive to the array I'd want it to
be given 50-block redundancy (thus making the 500GB drive not add any
usable space, but it would free up the 250GB drive, and future 500GB
drives would add space in a 1:1 fashion).  The largest drive in any RAID
like this has to be completely redundant since the smaller drives
obviously can't cover for it.

To me the biggest limitation of RAID-5/6/whatever is the fact that the
drives all have to be the same size.  Drives get bigger all the time -
if I want to add more space in a few years I won't want to do it by
trying to find some obsolete drive 1/10th the size of typical retail

The other pain with most RAID setups is the fact that it is a pain to
implement them using existing drives full of data.  If I wanted to setup
RAID-5 for my myth video I'd need to buy a whole stack of drives, set
them up, and then import my existing data into the new array.  Then I'd
have a whole stack of drives sitting around with no use (which I
couldn't even easily add to the new array).  In theory if I have 4x250GB
non-RAID drives I should just be able to add one more to get RAID-5 (and
not 5 more with 4 idle drives when done).

If somebody has a clever way to accomplish any of this let me know.  I'd
be interested in implementing it myself...

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