Hard Drive RAID -- 5 vs 6

Why would you pick RAID 5 if RAID 6
allows two drive failures instead of just one ?

http://www.adaptec.com/en-us/_common/compatibility/_education/raid_level_compar_wp.htm
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Carlos IjalbaIT Systems DirectorCommented:
Because RAID5 is a lot faster than RAID6, and with a few disks gives you more space.

This are the values in IOPs per 2 different RAID groups, using 6 HDs of 2TB:

TYPE        CAPACITY      IOPS
____________________________
RAID 6      7,298 GB      300
RAID 5      9,122 GB      395
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Paul MacDonaldDirector, Information SystemsCommented:
• RAID 6 is relatively new, so your controller may not support it
• RAID 5 gives you a better Disk Usage ratio (n-1 vs n-2)
• I'm not convinced by [Carlos Ijalba]'s statistics, but there may be some overhead due to having to calculate the additional parity bit.  If you're using a hardware controller that supports RAID 6, that overhead should be negligible however.
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Seth SimmonsSr. Systems AdministratorCommented:
RAID 6 is relatively new, so your controller may not support it

raid 6 has been around nearly 10 years

RAID 5 gives you a better Disk Usage ratio (n-1 vs n-2)

true; but doesn't prevent a double fault
i have seen people here experience that and experienced it once myself - lost the server
yes, there is overhead with the disk space loss but more reliable
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rindiCommented:
I'd never pick RAID 5 in any production environment. It is unreliable, and wrong manipulations can cause catastrophic failures. The only reason anyone would use RAID 5 is because you need less disks for as high a capacity as possible, so a decade or more ago, when server SCSI disks only came in very low capacities and were very expensive, it was a valid option. But not today with it's large disks and their low costs.
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Carlos IjalbaIT Systems DirectorCommented:
Raid 5 (Drives with Parity) — Minimum 3 Drives
Raid 6 (Drives with Double Parity) — Minimum 4 Drives

Raid5 has a write penalty of 4 ops per write
Raid6 has a write penalty of 6 ops (it has to write parity over 2 disks, not just one like Raid5)

You can read more about RAID penalty here:

http://theithollow.com/2012/03/understanding-raid-penalty/

In any case, you do use RAID5, RAID6, etc depending on what you need, space wise and speed wise. If you need speed go for RAID5, if what you need is reliability, RAID6.

The decision also depends if you have controller cache or SSD disks to do caching, etc.

I would go for RAID5, but in any case, check that you have a good backup policy and that you backup daily, whatever RAID you choose.
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DavidPresidentCommented:
The RAID6 performance penalty is a function of the specifics of your config and read/write mix.    Remember, a RAID controller can read disks in parallel, so there is no IOPs penalty on reads.

As for the write penalty, controllers that have write cache enabled and sufficient bandwidth can compensate for the additional workload to the extent that the only way you would notice R5 vs R6 write performance difference is if you are running a benchmark

As for the RAID5/6 minimum disk count .. techinically the answer is the minimum is 1 disk. You can do RAID5/6/1/0 on a single HDD if you do it on block ranges.   The adaptec hardware won't let you do that, but there is no specific requirement on number of disks to create any RAID level.    

In fact, you can do a RAID5 with just 2 disks on the adaptec.   This is what happens when you build a 3-disk RAID5 and one of the HDDs fail ;)

Just do a RAID6 and sleep at night.    Remember, 100% of disks fail ... eventually.
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Mohammed KhawajaManager - Infrastructure:  Information TechnologyCommented:
Depends on your hardware.  We have had servers as well as SANs where we had RAID-5 with multiple hot-spares where multiple drive failures did not lead to data corruption providing the failures were over time.
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Gerald ConnollyCommented:
I am with David (dlethe), RAID-6 is the safest way to go.

Carlos has a point that -5 needs less disks and has better utilisation, but as Rindi says, -5 was the king when disks were small, but in today's big disk era, the time to rebuild has got much longer, and while that -5 rebuild is going on, you are at risk from another disk failing ( probability medium to high) and then you are relying on your backups! Ie several hours/days downtime and potential data loss.
RAID-5 is not recommended for business use anymore
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