Re: What's the typical RAID10 setup?

[Roberto Spadim <roberto@spadim.com.br>]

ok, but lost of a disk = problem with hardware = big problems = mirror failed
think about a 'disaster recover' system
you can´t lost the main data (you MUST have one 'primary' data source)

raid1 don´t have ecc or anyother 'paged' data recover solution (it
have just all mirror resync)

let´s get back a level... (inside hard disk)
if your hard disk have 2 heads, you have a raid0 inside you disk (got
the point?)
using your math, you should consider head problem (since it make the
real read of information)

but at raid (1/0) software (firmware) level, you have devices (with
out without heads, can be memory or anyother type of adresseable
information souce, RAID0 = DEVICE for raid software/firmware,  but you
have A DEVICE)

for raid 1 you have mirrors(a copy of one primary device)
if software find 1bit of error inside this mirror(device), you lost
the full mirror, 1bit of fail = mirror fail!!!!! it´s not more sync
with the main(primary) data source!!!!

got the problem? mirror will need a resync if any disk fail (check
what fail make you mirror to fail, but i think linux raid1 mirror fail
with any disk fail)

if you have 4 mirrors you can loose 4 disks (1 disk fail = mirror
fail, 2 disk fail = mirror fail, 3 disk fail = mirror fail, any device
with fail inside a raid1 device will make the mirror to fail, got? you
can have good and bad disks on raid0, but you will have a mirror
failed if you have >=1 disk fail inside your raid0)

got the point?
what´s the probability of your mirror fail?
if you use raid0 as mirror
any disk of raid0 failed = mirror failed got?
you can lose all raid0 but you have just 1 mirror failed!


could i be more explicit? you can´t make probability using bit, you
must make probability using mirror, since it´s you level of data
consistency
=] got?


2011/1/31 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
> --
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-- 
Roberto Spadim
Spadim Technology / SPAEmpresarial
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