On Mon, 13 Jan 2014 11:15:13 +0100 Gionatan Danti [off-list ref] wrote:

On 01/13/2014 10:45 AM, NeilBrown wrote:

quoted

On Mon, 13 Jan 2014 09:52:50 +0100 Gionatan Danti [off-list ref] wrote:

quoted

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