[mythtvnz] File System for RAID-5

[Steve Hodge]

On Thu, Jun 18, 2009 at 6:47 AM, James Booth <james at booths.net.nz> wrote:

>
> I've set up a trial RAID 1+0 using Tim's layout as below, and it's all
> fine. I have one last question.
>
> Tim, using your setup, if I am running RAID 1+0 then will it be possible to
> add another disk later on without having to back all the data up first? (I'd
> try it, but don't have a spare disk to hand). Could I add in drive D as per
> your example, and then automatically grow the RAID 0 across the top? I have
> not been able to find a definitive answer on this on the net.
>
> Same goes for RAID10,F2 - if I go this route can I just automatically grow
> the system with an extra disk later on?
>

As far as I know you can't grow raid10 arrays. It's been on the todo list
for at least a couple of years but I haven't found anything to indicate that
it's been implemented. Personally I've found that by the time I want to
expand an array the best value disks are significantly larger than the disks
in the array. So a simple expand would leave me with part of the new disk
left over. So I find I want to either rebuild from scratch anyway or setup
the new disk in a second array. Usually if I think about it long enough I
can figure out a way to add the new drive without having to back anything up
by degrading the existing array to free up space.

Here's an example of the sort of thing I've done in the past: Say I started
with 3 500GB drives in a raid10 (A, B, and C, total size 750GB). I buy a new
1000GB drive (D). I partition the new drive into 2 500GB partitions - you
really need to be dealing with equal sized partitions. I can't make a single
raid10 out of the 5 500GB partitions unless I set it up with 3 copies of
everything because if there are only two copies then some blocks will have
their two copies on D1 and D2 which is not safe. Of course if I rebuilt the
whole array I'd have to find temporary space for 750GB of data and I don't
have that available anyway. So what I do instead is to replace one of the
existing partitions in the raid10 with one of the new partitions:
1. degrade the array by removing (say) A.
2. add D1 to the array and let it resync.
Now I have A and D2 to make a raid1. I can either use that as a separate
filesystem or, if I was using LVM over the original array I can combine the
two arrays that way.

There are two obvious alternatives to this sort of thing. First, you could
use raid5 which is growable. Just be aware that it degrades to a raid0 - I
don't recommend expanding a raid5 beyond about 4 disks. If you're expecting
to end up with more than that then raid6 would be a safer option. The second
alternative is to always expand by pairs of disks. If you do that then it's
trivial to set up new raid1 mirrors and combine them with existing arrays
using LVM.

Cheers,
Steve
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