[CentOS] OT: What's wrong with RAID5

[Peter Kjellstrom]

On Thursday 24 September 2009, Fajar Priyanto wrote:
> Hi all,
> Sorry for the OT.
> I've got an IBM N3300-A10 NAS. It runs Data Ontap 7.2.5.1.
> The problem is, from the docs it says that it only supports either
> RAID-DP or RAID4.
> What I want to achieve is Max Storage Capacity, so I change it from
> RAID-DP to RAID4, but with RAID4, the maximum disk in a RAID Group
> decrease from 14 to 7. In the end, either using RAID-DP or RAID4, the
> capacity is the same.

Both raid4 and raid5 could thoretically be used with 14 drives. Why they limit 
you to 7 drives at all is a good question (maybe you should ask IBM?). 
Possibly they consider too large arrays with only a single drive worth of 
parity un-safe.

> Now, why RAID5 is not supported? I believe using RAID5, I can get more
> storage capacity, can't I?
> I also notice with some onboard RAID controller, they only support
> either RAID0, RAID1, or RAID1+0. No RAID5.

This has a completely different explanation. RAID0, 1 or 1+0 is a "simple" 
case of juggling sectors, no parity engine is needed. RAID4, 5 or 6 would 
require a much more complex and powerfull design.

> What's wrong with RAID5, is there any technical limitation with RAID5?

Compared to raid4: not much at all
Compared to raid10: less safe, longer rebuilds, slower
Compared to raid6: less safe, more usable space, typically faster

/Peter
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