On 01/13/2014 10:45 AM, NeilBrown wrote:
On Mon, 13 Jan 2014 09:52:50 +0100 Gionatan Danti <g.danti@xxxxxxxxxx> 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.
I am wrong?
Regards.
--
Danti Gionatan
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Assyoma S.r.l. - www.assyoma.it
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