[geeks] recommended dual channel LVD controller, PC

[Scott Howard]

On Fri, Jul 23, 2004 at 12:50:41PM -0400, Sandwich Maker wrote:
> because if you stripe mirrors, any one failed mirror takes you down,
> but you can lose an drive on each mirror and stay up.
> 
> for your setup of just 4 disks, it -is- moot.  but when you stripe
> more than two mirrors...

No it's not! It's moot for a 2 disk setup (which obviously can only me
a straight mirror) but for anything beyond that the 1+0 v's 0+1 argument
is valid.

> i've heard that disksuite silently reorgs mirrored stripes into a
> striped mirror.

Sorta... It actually does mirrored stripes, but in the event of a failed
disk it will not offline the entire stripe - it just keeps track of
which parts of the stripe are good and which are bad, and only reads
from the good parts.
This actually gives you best of both worlds - you get the resilience of
1+0, but you get the flexibility of 0+1, such as being able to split off
a mirror to use for backups, etc.

The Sun T3 arrays do mirroring in an even more weird way. All data on a
disk is mirrored to the disk beside it, but it's done at the block
level rather than the disk level.  So with 6 disks you get
something like :

Disk1   Disk2   Disk3   Disk4   Disk5   Disk6
AAAA    AAAA    BBBB    BBBB    CCCC    CCCC
DDDD    DDDD    EEEE    EEEE    FFFF    FFFF
GGGG    GGGG    HHHH    HHHH    IIII    IIII

So in this setup you can lose multiple disks, as long as they are not
next to each other, but at the end of the day it's really just RAID 1+0.
The real advantage of this method is that you can actually setup a 3-disk,
2-way mirror. ie, If you've got 3x36Gb disks then you can setup a 2-way
50Gb mirror, with the data layed out like :

Disk1   Disk2   Disk3
AAAA    AAAA    BBBB
BBBB    CCCC    CCCC
DDDD    DDDD    EEEE
EEEE    FFFF    FFFF

Of course, you can do this with software mirroring by breaking the disk
up into multiple partitions/subdisks and then concat'ing them, but it's
not half as neat as how the T3 does it.

  Scott