Re: What's the typical RAID10 setup?

["Keld Jørn Simonsen" <keld@keldix.com>]

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:
> 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
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-raid" in
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> >
> 
> 
> 
> -- 
> Roberto Spadim
> Spadim Technology / SPAEmpresarial
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