experiences with RAID 6

[Kaiser, Hans]

> >> RAID6 is a standard!
> > 
> > RAID6 is a standard in the same way that the OpenDocument Architecture 
> > is a standard. Sure, it's clearly defined and documented, but it's not 
> > something that a lot of people do in real life. Most RAID systems are 0, 
> > 1 and 5 and it's a good idea to stick to those.

Raid 0 is only for performance reasons a good choice. If you use it,
you're reducing availability massively!
Pro:
- max. available diskspace is used
- max. performance
con:
- every added drive can destroy the whole set of data in the RAID0. The
more drives the more risk


> >> RAID 6 is a better solution as RAID 5 and RAID 1!
> >> Both Systems can only live with 1 defect HDD.
> > 
> > RAID1 can work with arbitrarily many defect hard disks. Because it's 
> > mirrored, if you have 10 disks, you can afford to have 9 of them go bad 
> > and still have the system work.

 ACK, but

con:
- RAID1 uses only the diskspace of the smalles drive in the set. e.g. 5x
20GB and 5x100GB => max available for RAID1 20GB!
  or 10x 100GB => max 100GB instead of 1000GB
pro:
- minimum risk of data loss, but only if used with more sparedrives than
RAID5 and RAID6!


> >> RAID 6 can work with 2 defect HDDs!
> > 
> > Do you, in practice, know how to recover a RAID 6 system with two defect 
> > HDDs? RAID 6 is a very non-trivial setup. That's why it's not used much.

Sure, with hotspare replace the defect disc, without add a new disk and
the system rebuilds it...
The Setup is not easy in linux, but RAID5 and RAID6 are no RAIDs, which
should be used in Software! They are definitively to slow.


> >> Therefore it is a good question, if someone has experiences in RAID 6 
> >> for HA systems
> > 
> > I didn't say it was a bad question. I suggested that you stick to things 
> > that are common and well tested and asked if you had a compelling reason 
> > to not follow the most tested route.

RAID6 is also good tested. In really big environments (~30TB with 25k
IO/s) already commonly used.

> > How many disks do you plan to have? What do you want to use this 
> > computer for?

I expect, if someone ask for RAID6, that the person nows, that RAID6
would be expensive (CPU-Power or money).
Anyway it is a good question. RAID6 is only a good choice if you plan a
RAIDset of at an absolute min of at least 5 drives. Else it is better to
use RAID10 (stripe + mirror). Fewer CPU-Power is needed and therefore
really cheap


> > Cheers,
> > Daniel.
> 
> 	Plus there's the fact that RAID 6 is inefficient with a small number of 
> drives. It's 's only good when you have something like 10 drives or more 
> you want to put in a RAID setup.


10 drives is not really needed 5 are enough to be efficient. 4 drives
are compareable to RAID10, which are already very inexpensive (3ware
8000 series ~100EUR/$)

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