2011/1/31 Roberto Spadim [off-list ref]:

i think that partial failure (raid0 fail) of a mirror, is a fail
(since all mirror is repaired and resync)
the security is, if you lose all mirrors you have a device
so your 'secure' is the number of mirrors, not the number of disks ssd
or another type of device...
how many mirrors you have here:
raid0= 1,2(a) 3,4(b)
raid1=a,b
1 mirror (a or b)

and here:
raid1=1,2(a) 3,4(b)
raid0=ab
1 mirror (a or b)

let´s think about hard disk?
your hard disk have 2 disks?
why not make two partition? first partition is disk1, second partition is disk2
mirror it
what´s your security? 1 mirror
is it security? normaly when a harddisk crash all disks inside it
crash but you is secury if only one internal disk fail...

that´s the point, how many mirror?
the point is
with raid1+0 (raid10) we know that disks are fragments (raid1)
with raid0+1 we know that disks are a big disk (raid0)
the point is, we can´t allow that information stop, we need mirror to
be secured (1 is good, 2 better, 3 really better, 4 5 6 7...)
you can´t break mirror (not disk) to don´t break mirror have a second
mirror (raid0 don´t help here! just raid1)

with raid10 you will repair smal size of information (raid1), here
sync will cost less time
with raid01 you will repair big  size of information (raid0), here
sync will cost more time

Roberto, to quite understend how better a raid 10 is over raid 01  you
need to take down into a mathematical level:

once I had the same doubt:

"The difference is that the chance of system failure with two drive
failures in a RAID 0+1 system with two sets of drives is (n/2)/(n - 1)
where n is the total number of drives in the system. The chance of
system failure in a RAID 1+0 system with two drives per mirror is 1/(n
- 1). So, for example, using a 8 drive system, the chance that losing
a second drive would bring down the RAID system is 4/7 with a RAID 0+1
system and 1/7 with a RAID 1+0 system."


Another problem is that in the case of a failury of one disk ( in a
two sets case), in a raid01 you will loose redundancy for ALL your
data, while in a raid10 you will loose redundancy for 1/[(n/2
-1)/(n/2)], in the same case 1/4 of your data set.

And also, in a raid 10 you will have o re-mirror just one disk in the
case of a disk failure, in raid 01 you will have to re-mirror the
whole failed set.

-- 
Denis Anjos,
www.versatushpc.com.br
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