Data Centers

RAID 5 or RAID 6: Which should you select?

Many RAID controllers now support both RAID 5 and RAID 6. IT pro Rick Vanover explains what RAID 6 is and when to select it over RAID 5.

When it comes to architecting a storage solution, planning is key. In addition to selecting the storage protocol (iSCSI, fibre channel, NAS, etc.) and disk type to use (SAS, SATA, SSD, etc.), you should also give a lot of thought about what RAID algorithm to use.

I used to try to keep it simple and stick to either RAID 1 or RAID 5 when I wasn't working with larger amounts of storage. However, when it comes to provisioning larger storage in any SAN or NAS environment, it may be worth selecting RAID 6 over RAID 5 for larger arrays. To help you understand how RAID 5 and RAID 6 differ, we'll explain each RAID type.

RAID 5 is an array that has a distributed parity bit across the array. Figure A is a representation of RAID 5. Figure A

RAID 5

The grey blocks are the parity bit through the array. RAID 5 takes a minimum of three drives to implement; this example uses four drives to give an easier visual comparison to RAID 6.

RAID 6 uses two independent parity schemes that maintain array integrity. Figure B represents RAID 6. Figure B

RAID 6

The grey blocks are the parity bits, but the two parity algorithms exist separately on their blocks. RAID 6 has more overhead in terms of usable storage compared to the raw amount, as well as a more complex RAID controller algothrithm. According to the AC&NC RAID information sheet, RAID 5 has better write performance.

I prefer building for RAID 6 in spite of the RAID 5 write performance advantage. This is primarily due to the rebuild times being less impactful on disk with RAID 6 even though they take longer, which is an important factor considering we now have 2 TB or larger disks in use in many NAS and SAN storage systems. RAID 6 has additional protection against block failures and controller errors by the extra parity. If zero loss of data is a priority, this is something to consider. I've had controllers have logic errors and data loss even when the actual disks are healthy.

What is your take on RAID 5 vs. RAID 6? Share your comments below.

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About

Rick Vanover is a software strategy specialist for Veeam Software, based in Columbus, Ohio. Rick has years of IT experience and focuses on virtualization, Windows-based server administration, and system hardware.

71 comments
StithDehere
StithDehere

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jmjosh
jmjosh

For all those who claim it's better to have RAID10 (or 1+0, or 0+1) than RAID6:

If you have the basic config with 4 drives - RAID10 wins with higer speeds but RAID6 wins with higher security (ANY 2 disks can fail, and in RAID10 2 failed disks can kill your data).

Once we start increasing the number of drives - RAID10 still wins with write/read speed, but still can fail if 2 disks from one mirror fail. This becomes significant, since the more drives we have - the probability of a disk failure increases. And since it's rather hard to get - let's say - 40 drives, each from different production series, the probability of 2 disks failing at the some time is quite high.

At the same time, space efficiency falls dramatically - with 12 4TB disks we have 40TB space for RAID6 and 24TB for RAID10.

So, I'd rather go for RAID6 with some hotspare in order to have reasonable speed and data security.

Actually, I wish there were some RAID-6 like plans for bigger arrays, that would protect from 5-6 disks failing at the same time.

If I am about to create array from 100 4TB drives, I'd rather split it to 10 RAID-6 arrays giving 320TB to reduce risk of losing all data when 2% of drives somehow fail, at cost of 20% overall drive capacity. If there was a RAID plan that would consume 10% of capacity, that would give untouched data even when 10 disks fail, what would be great - without need to split data etc.

ganesh.borhade
ganesh.borhade

Hello Rick, What is impact on file server (read) when we use RAID6 (in place of RAID5) ? How to compensate write performance issue of RAID6? Can we add new disks? Regards Ganesh

vivekgreets
vivekgreets

When you say large storage for RAID6 how much storage do you refer to? I would like to have an idea of the large storage range please.

drsw
drsw

The company for which I work has thousands of backup appliances in the field from which we get a tremendous amount of data concerning this subject. After years using RAID-5, we now ship only RAID-1 in our low-end appliances, RAID-10 in our medium-end appliances, and RAID-6 (actually, RAID-1/RAID-6) in our higher-end appliances. In the real world, with larger disks, the major reason that you end up going to RAID-6 over RAID-5 is the concern with respect to drive failure during in-operation rebuild. __________________ Mark Campbell http://www.unitrends.com/

nick.ferrar
nick.ferrar

I don't understand the reasoning in the original article, why sacrifice performance to reduce disk rebuild times (a rare occurrence)? If you need more redundancy and quicker rebuilds than RAID 5 you should be using RAID 10, most storage vendors I've discussed it with hate the idea of RAID 6 - they just had to add support for it because their competitors supported it and poorly informed end users bought into the smoke and mirrors.

jeslurkin
jeslurkin

Glad to learn of newer RAID configs.

Rick_hayward99037
Rick_hayward99037

Many times, the choice of RAID comes down to software and hardware support. When RAID-6 isn't supported, it becomes a choice of "use what you have" VS "buy a new controller card." On our last non-recoveable RAID-5 experiance, we opted to do RAID-10. Our controller card could do it, but not RAID-6, and we would have enough storage in that configuration, with the hard drives we had. RAID-10 is very redundant, so very hard to kill. It also could be considered wasteful, but drives are cheap too. The actual read/write speeds will depend on the controller card's processor, and hard drives used.

jjbueyes
jjbueyes

RAID 5 is better when used on Database systems, and may be less expensive

ScarF
ScarF

Drawing the correct image for RAID-5 with 4 disks, eh? The distributed parity requires 1/N space on each disk, where N is the number of disks in array. Same remark for RAID-6. Since you have 4 disks, each disk is divided in 4 areas, only. Otherwise, the advantage of using RAID-6 for large ammounts of data, is correct.

Leo
Leo

Considering that I'm not going to be rebuilding disk more often that I will need to write to them, i will still have to choose RAID5. Performance is more important to me than rebuild times. Plus, the added cost just isn't justified. -Leo

dr.funkenstein01
dr.funkenstein01

Hi, Has anyone used any of the DROBO products. What you're views on DROBO

antermyhome
antermyhome

I think, RAID 5 is better for smaller Companies where data priority is less then data availabilty. Other side RAID 6 is better for large data storage, where every data is critical.

rajesh.pillai
rajesh.pillai

With disks having TB of capacity, the rebuild time also increases, RAID 6 can reduce this risk

klapper
klapper

Agreed, we've got a Promise vTrack with 16 1TB drives in a RAID6 configuration. We do lose some disk space and there is probably a slight performance hit; but the rebuild and fault tolerance benefits make it worth it. These 16 drives were manufactured very close to each other. If one drive dies, we think the probably of a second drive dying increases. RAID6 helps with that issue.

jschlaf1
jschlaf1

Recently had a 5+ TB array in raid 5 for archiving data. The write performance was terrible and rebuild took several days. In addition GUID Partition table not recognized by older clients. I replaced it with multiple arrays of RAID1. still have the rdundancey and better all around performance.

GDF
GDF

The way I remember this (and it's been a while), with RAID 5 you need one extra drive, and with RAID 6 you need two. So for large arrays the difference in overhead (i.e., drives purchased vs. storage available) is not significant. Then in the case of a drive failure - which is still the most common failure mode for RAIDs - with RAID 5 you're running unprotected and need to get a replacement drive in "right now"; with RAID 6 you still have RAID-5-equivalent protection and can replace the failing drive on a more practical schedule.

renodogs
renodogs

You lightly touched on one major aspect of today's drives and inherent problems within: error correction on perfectly good drives. Simply put, there's a massive amount of overhead on today's drives given the manufacturer's acceptable hardware tolerance for error correction. Read your drive specs, I think you'll be surprised. The rule of thumb is the larger drive space you have, the larger allotted error correction time; which = cpu peformance overhead. Life's great when we can store the equivalent of the Library of Congress on a few arrays. Nonetheless, one cannot simply ignore the data errors that exist on perfectly functioning drives. IT folks that don't factor in the leakage of errors are doomed to long hours of chasing the blame game from the upper echelon screaming "I thought this system was going to be reliable!!!" Well Sir, it is, sort of. Factor in aging drive spindle noise, worn internal mechanical components of the drive- and you have all the makings of a king size headache. The bottom line is this: in any disk array, there are trade-offs in performance, integrity, and finally, cost. Therefore, the reality of which array and which architecture to select must come down to two things: first, data integrity. Just how good is it? Secondly, cost. You can't ignore the CFO and their insistence to remain within a sane budget. Anything less than that and you will fall into that age old trap of 'head in sand' viewpoint, and it WILL come back to bite you.

phatty
phatty

RAID 6 rebuild times are slower than RAID 5. This article is incorrect.

byoung
byoung

If zero data loss is the only goal, then RAID 6 is more efficient than RAID 1, which has 50% overhead. But where 100% uptime is required, you still can't beat RAID 1. Perhaps a RAID 1+6 solution, while expensive, is the best for large storage systems.

feenberg
feenberg

I would be interested in learning why rebuild times should be faster with Raid 6. It doesn't seem obvious, as the rebuild process still has to read or write the same number of drives with raid 5 or raid 6 - doesn't it?

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