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On Wednesday 16 June 2010 04:43:06 martin f krafft wrote:
> also sprach Boyd Stephen Smith Jr. <bss@iguanasuicide.net> [2010.06.15.2108 
> > > Use mdadm for a RAID5 or RAID6 and LVM on top for the remaining
> > > cases when you need space and care less about performance.
> > 
> > Use RAID 1/0 in mdadm when you need redundancy, space, and performance.
> > 
> > (Although, IME, RAID 5 is not badly performing.)
> Sure it is, on writes. If you have the right hardware, you won't
> notice, but every write takes twice as much work.
> http://git.debian.org/?p=pkg-mdadm/mdadm.git;a=blob;f=docs/RAID5_versus_RAI
> D10.txt;hb=refs/heads/contrib/docs/raid5-vs-raid10

While disks may have gotten cheaper, some people only need 20% (or less, RAID 
5 doesn't have to be 5 disks) redundancy instead of the 50%, 67%, or more 
redundancy provided by RAID 1/0.  (Also, RAID 1/0 in mdadm is not just stripes 
over mirrored pairs OR mirroring over striped sets.)

RAID 1/0 does indeed have all the recovery advantages mentioned, and I 
generally recommend it when you can afford the 50% redundancy.

The support for RAID 3 or 4 over RAID 5 at the end is completely without 
basis, and almost certainly wrong.  RAID 5 performs better than RAID 4 and 3 
under all conditions and has the same failure cases.  Clearly, the author was 
biased against RAID 5 from the start, which throws suspicion on the anecdotes 
in the prose.

Still, the issues listed about RAID 5 recovery are very real, and should be 
enough to make one reconsider either spending more to be able to use RAID 1/0 
and have the same usable space or using RAID 1/0 and doing with less usable 

> http://git.debian.org/?p=pkg-mdadm/mdadm.git;a=blob;f=debian/FAQ;hb=HEAD#l
> 106

The layout used with near=2 is, of course, exactly the same as layering.  It 
is unclear, then, how this can be applied to an odd number of devices, per the 
man page: "e.g. it is perfectly legal to have an ’n2’ layout for an array with 
an odd number of devices"

I was under the impression that offset=2 would do better about rotating which 
pairs were mirrored for a data block.  The diagrams show hdd1 being mirrored 
with hdd2 and hdd4 but not with hdd3; similar with far=2.
Boyd Stephen Smith Jr.           	 ,= ,-_-. =.
bss@iguanasuicide.net            	((_/)o o(\_))
ICQ: 514984 YM/AIM: DaTwinkDaddy 	 `-'(. .)`-'
http://iguanasuicide.net/        	     \_/

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