Re: RAID 10 far and offset on-disk layouts

[NeilBrown <neilb@suse.de>]

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On Mon, 13 Jan 2014 11:15:13 +0100 Gionatan Danti <g.danti@assyoma.it> wrote:

> On 01/13/2014 10:45 AM, NeilBrown wrote:
> > On Mon, 13 Jan 2014 09:52:50 +0100 Gionatan Danti <g.danti@assyoma.it> wrote:
> >
> >> Hi Neil,
> >> let me recap from a previous message:
> >>
> >>   >FAR LAYOUT
> >>   >md(4) states:
> >>   >"The first copy of all data blocks will be striped across the early >part
> >>   >of all drives in RAID0 fashion, and then the next copy of all blocks
> >>   >will be striped across a later section of all drives, always ensuring
> >>   >that all copies of any given block are on different drives"
> >>   >
> >>   >The "on different drives" part let me wonder _how_ are chunks
> >>   >distributed. On a 4-disk array, I can imagine some different schemas:
> >>   >
> >>   >1)	A1 A2 A3 A4
> >>   >	.. .. .. ..
> >>   >	A4 A1 A2 A3
> >>   >
> >>   >2)	A1 A2 A3 A4
> >>   >	.. .. .. ..
> >>   >	A2 A1 A4 A3
> >>   >
> >>   >The first schema is the one depicted by SuSe documentation [1], while
> >>   >the second is the one described by Wikipedia [2].
> >>   >
> >>   >Question 1: as the two schema have different reliability
> >>   >characteristics, which is really used?
> >>
> >> SuSe entry:
> >> https://www.suse.com/documentation/sles11/stor_admin/data/raidmdadmr10cpx.html#b7cynnk
> >>
> >> Wikipedia entry:
> >> http://en.wikipedia.org/wiki/Linux_MD_RAID_10#LINUX-MD-RAID-10 (see how
> >> far layout is depicted)
> >>
> >> Keld kindly told me that the SuSe is simply not updated, as it depict a
> >> situation changed with newer kernels. So my two questions:
> >
> > I cannot see an important difference between the two pages you reference.
> > Both appear to be correct.
> 
> Mmm... they seem different to me.
> 
> SeSe FAR Layout:
> 
> sda1 sdb1 sdc1 sde1
>    0    1    2    3
>    4    5    6    7
>    . . .
>    3    0    1    2
>    7    4    5    6
> 
> Notice how (for example) sdb1 is coupled both to sda1 (0,4) and 
> sdc1(1,5). If sdb1 fails, any sda1 or sdc1 failure lead to data loss.
> 
> Now, Wikipedia FAR Layout:
> 
> 4 drives (sda1, sdb1, sdc1, sdd1)
> --------------------
> A1   A2   A3   A4
> A5   A6   A7   A8
> A9   A10  A11  A12
> ..   ..   ..   ..
> A2   A1   A4   A3
> A6   A5   A8   A7
> A10  A9   A12  A11
> ..   ..   ..   ..
> 
> Notice now how a single disk (eg: sdb1) is coupled to only another 
> _single_ disk (eg: sda1). In this case, if sdb1 fails, you had to lose 
> sda1 to have a data loss. Losing sdc1 or sdd1 will _not_ lead to data loss.
> 

Thanks for being explicit - it is much easier to answer explicit questions :-)

Yes, they are different.  So the wikipedia article is wrong, or at least
misleading.  That is not what the "f2" layout looks like.

The md driver does support that layout.  I don't know yet what mdadm will
call it, but it won't be called "f2".

So this change:

http://en.wikipedia.org/w/index.php?title=Non-standard_RAID_levels&diff=501908270&oldid=501604733

was wrong.

NeilBrown

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