Re: What's the typical RAID10 setup?

[Denis <denismpa@gmail.com>]

2011/1/31 Roberto Spadim <roberto@spadim.com.br>:
> 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
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html