Re: [gentoo-amd64] Re: Is my RAID performance bad possibly due to starting sector value?

[Rich Freeman <rich0@gentoo.org>]

On Sat, Jun 22, 2013 at 6:29 AM, Duncan <1i5t5.duncan@cox.net> wrote:
> Rich Freeman posted on Fri, 21 Jun 2013 11:13:51 -0400 as excerpted:
>> If you protect 1 drive of data with 25 drives of parity (call them
>> mirrors or parity or whatever - they're functionally equivalent) then
>> you need 25/26 drives to fail to lose 1 drive of data.
>
> Almost correct.

DOH - good catch.  Would need 26 fails.

> AFAIK 13 drives of data with 13 mirrors wouldn't (normally) be called
> raid1 (unless it's 13 individual raid1s)...

That's why I commented that I find RAID "levels" extremely unhelpful.
There is striping, mirroring, and RS parity, and every possible
combination of the above.  We have a special name raid5 for striping
with one RS parity drive.  We have another special name raid6 for
striping with two RS parity drives.  We don't have a special name for
striping with 37 RS parity drives.  Yet, all three of these are the
same thing.

I was referring to 13 data drives with one mirror each . If you lose
two drives you could potential lose one drive of data.  If you made
that one big raid10 then if you lose two drives you could lose 13
drives of data.  Both scenarios involve bad luck in terms of what pair
goes.

> You're right that at that level, you DO need a real backup, and it should
> take priority over raid-whatever.  HOWEVER, in addition to creating a
> SINGLE raid across all those drives, it's possible to partition them up,
> and create multiple raids out of the partitions, with one set being a
> backup of the other.

I wouldn't consider that a great strategy.  Sure, it is convenient,
but it does you no good at all if your computer burns up in a fire.

Multiple-level redundancy just seems to be past the point of
diminishing returns to me.  If I wanted to spend that kind of money
I'd probably spend it differently.

However, I do agree that mdadm should support more flexible arrays.
For example, my boot partition is raid1 (since grub doesn't support
anything else), and I have it set up across all 5 of my drives.
However, the reality is that only two get used and the others are
treated only as spares.  So, that is just a waste of space, and it is
actually more annoying from a config perspective because it would be
really nice if my system could boot from an arbitrary drive.

Oh, as far as raid on partitions goes - I do use this for a different
purpose.  If you have a collection of drives of different sizes it can
reduce space waste.  Suppose you have 3 500GB drives and 2 1TB drives.
 If you put them all directly in a raid5 you get 2TB of space.  If you
chop the 1TB drives into 2 500GB partitions then you can get two
raid5s - one 2TB in space, and the other 500GB in space.  That is
500GB more data for the same space.  Oh, and I realize I wrote raid5.
With mdadm you can set up a 2-drive raid5.  It is functionally
equivalent to a raid1 I think, and I believe you can convert between
them, but since I generally intend to expand arrays I prefer to just
set them up as raid5 from the start.    Since I stick lvm on top I
don't care if the space is chopped up.

Rich