JP Vossen on 30 Apr 2008 15:50:19 -0700

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Re: [PLUG] RAID @ Home (was "Saving data from a precarious drive")

 > Date: Wed, 30 Apr 2008 17:45:27 -0400
 > From: "Brian Vagnoni" <>

 > I always though,t perhaps incorrectly that you could do a software
 > RAID 5 with linux? Am I wrong about that, please and sincerely let
 > me know?

You are not wrong, I guess I should have been more clear about that.

You certainly CAN do RAID5 with Linux (and probably BSD and others, 
though I've never checked).  But RAID5 calculations cause a performance 
hit, which is smaller when the work is offloaded to a dedicated RAID 
card.  I think that hit is the major reason why some people don't like 
software RAID5.  OTOH, with software RAID5 you are *not* tied to a 
physical RAID card that may die and leave your data stranded.

Software mirroring doesn't have the same performance hit as software 
RAID5, and with the price of disks today, the 0.5 vs. N-1 space hit is 
more tolerable.  To elaborate on that, with any kind of RAID5, you 
require 3 or more disks, but you get N-1 of the space you bought, e.g. 
100G x 3 = 300G, but you can only use 200G as 1 disk is for parity 
(oversimplified).  With mirroring, you require 1 or more pairs disks, 
but you lose half the capacity, so 100G x 2 = 200G but you can only use 

JP Vossen, CISSP            |:::======|        jp{at}jpsdomain{dot}org
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