Re: What's the typical RAID10 setup?

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

On Mon, Jan 31, 2011 at 05:35:31PM -0200, Roberto Spadim wrote:
> 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

you are confused, please read below.

> 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?

My question was really not related to your question, it is a general
question for the design of Linux MD RAID10.

And please keep terminology clean. RAID10 here on the list is Linux MD
RAID10. This is very different from what was called RAID10 five years
ago. The term for that is RAID1+0, meaning that you have 2 RAID1
devices, and then you make a RAID0 over the 2 RAID1 devices.

Best regards
keld


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