Thursday, July 16, 2009

RAID 10 Setup {"far","offset","near"}

Would love to research into the RAID 10 Setup's "far","near","offset".

As per the discussion at This is an summary of the RAID Layouts "far","offset" and "near" from an classic RAID 10 setup as discussed in

As quoted by Neil Brown at
On Thursday October 5, madduck [at] wrote:
> If A,B,C are data blocks, a,b their parts, and 1,2 denote their
> copies, the following would be a classic RAID1+0 where 1,2 and 3,4
> are RAID0 pairs combined into a RAID1:
> hdd1 Aa1 Ba1 Ca1
> hdd2 Ab1 Bb1 Cb1
> hdd3 Aa2 Ba2 Ca2
> hdd4 Ab2 Bb2 Cb2
> How would this look with the three different layouts? I think "near"
> is pretty much the same as above, but I can't figure out "far" and
> "offset" from the md(4) manpage.

near=2 would be

hdd1 Aa1 Ba1 Ca1
hdd2 Aa2 Ba2 Ca2
hdd3 Ab1 Bb1 Cb1
hdd4 Ab2 Bb2 Cb2

offset=2 would be
hdd1 Aa1 Bb2 Ca1 Db2
hdd2 Ab1 Aa2 Cb1 Ca2
hdd3 Ba1 Ab2 Da1 Cb2
hdd4 Bb1 Ba2 Db1 Da2

far=2 would be
hdd1 Aa1 Ca1 .... Bb2 Db2
hdd2 Ab1 Cb1 .... Aa2 Ca2
hdd3 Ba1 Da1 .... Ab2 Cb2
hdd4 Bb1 Db1 .... Ba2 Da2

Where the second set start half-way through the drives.The advantage of far= is that you can easily spread a long sequentialread across the drives. The cost is more seeking for writes.offset= can possibly get similar benefits with large enough chunksize, though I haven't tried to understand all the implications ofthat layout. I added it simply because it is a supported layout in DDF and I am working towards DDF support.

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