On 01/13/2014 11:27 PM, NeilBrown wrote:
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
Ok, so let recap:
1) FAR layout is the one depicted by SuSe documentation, while the
Wikipedia entry is wrong
2) MD _can_ produce a FAR layout as depicted by Wikipedia, but we don't
know how the user-space mdadm tool call it (maybe it is not implemented
yet?)
3) There are any reasons why FAR and OFFSET layout scramble data in this
manner, coupling any disk with two more disks? It was done for
simplicity, or I am missing something?
4) you confirm that currently we can _not_ create a FAR layout as the
one depicted by wikipedia by no means? What about OFFSET layout?
Thank you very much.
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