the main line of how to calcule the probability here:
YOU CAN´T HAVE A LOST OF INFORMATION!
so you can´t allow the MIN(probability to fail) be the secured
probability you MUST use the MAX(probability to fail)
MAX(probability to fail) = 1 disk failed = 1 mirror failed got?

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

the question is:
how many mirrors you have? you don´t have a partial mirror (i didn´t
found it on raid documentation yet), or you have a working mirror or
you don´t have the mirror and must resync to have a running one

raid10 = raid1
but the raid1 devices are raid0
if you put raid1 over raid0 or raid0 over raid1 is not a diference of
security. just a diference of how many time i will wait to resync the
raid1 mirror (a big raid0 you slower than smallers harddisks/ssd
devices)

the question again:
how many mirrors you have?

2011/1/31 Keld Jørn Simonsen [off-list ref]:

quoted

Top-posting...

How is the raid0+1 problem of only 33 % survival for 2 disk with RAID10?

I know for RAID10,F2 the implementation in Linux MD is bad.
It is only 33 % survival, while it with a probably minor fix could be 66%.

But how with RAID10,n2 and RAID10,o2?

best regards
keld


On Mon, Jan 31, 2011 at 05:15:29PM -0200, Roberto Spadim wrote:

quoted

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 [off-list ref]:

quoted

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

quoted

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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