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How many drives can fail in a RAID-DP netapp filer aggregate

Posted on 2014-11-01
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Last Modified: 2016-12-08
Hi we recently had 2 drives fail.

The RAID still held - and still appeared to have 2 parity drives.

question: could I have managed to have a 3rd drive fail - without collapsing or losing data?
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Question by:philb19
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15 Comments
 
LVL 7

Expert Comment

by:Stampel
ID: 40417021
To answer correctly i need to know :
How many disks you had/have ?
What RAID level are you using ? RAID5/RAID50/RAID6/RAID60/RAID10 ?
Did you have hotspares ?

For example in a RAID6 with 12 disks (that would do a single RAID0 of three RAID6 of 4disks each) you could lose maximum six of the 12 total disks (two in each set if you are lucky) and it could also fail with only 3 bad disks (3 in the same set) if you are very unlucky.

you may have a look at those links :
http://www.techrepublic.com/blog/the-enterprise-cloud/raid-50-offers-a-balance-of-performance-storage-capacity-and-data-integrity/
http://www.techrepublic.com/blog/the-enterprise-cloud/understand-when-raid-60-is-overkill/
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LVL 37

Expert Comment

by:Neil Russell
ID: 40417023
Did a hot spare kick in and the raid rebuild to that?
A triple disk failure in any single raid group is terminal. You would then need to be spending big bucks to get your data recovered or of course going back to that very reliable backup that you keep :D
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LVL 37

Expert Comment

by:Neil Russell
ID: 40417024
@Stampel
Please read the question.

This is a NETAPP RAID DP implementation so what you say is not relevant.  There is no " RAID5/RAID50/RAID6/RAID60/RAID10 ?"
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Expert Comment

by:Stampel
ID: 40417027
RAID-DP uses RAID6 and only prevents from the loss of 2 disks.
A 3rd failed drive would have collapsed the volume
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LVL 37

Expert Comment

by:Neil Russell
ID: 40417030
RAID DP Is a proprietary implementation and not a standard raid 6 implementation.  I already stated that it is two disks.

If you wish to read the technical explinations, see here

http://community.netapp.com/t5/Tech-OnTap-Articles/Back-to-Basics-RAID-DP/ta-p/86123
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Expert Comment

by:Stampel
ID: 40417038
Read again you did not state its RAID6, i did :)
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LVL 37

Expert Comment

by:Neil Russell
ID: 40417041
No i did not say it was raid six as it is not. It is RAID DP, as stated by the questioner.  I did state that a triple disk failure would be terminal.

The objective here is not to argue about specifications it is to give the questioner clear concise accurate information based on reading his/her question.  My first reply did so.
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Expert Comment

by:Stampel
ID: 40417053
You are wrong, RAID DP uses RAID6 check the documentation !
http://www.netapp.com/us/products/platform-os/raid-dp.aspx
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Expert Comment

by:rindi
ID: 40417056
As stated already it is RAID DP, which is a special form or RAID 6. Raid 6 uses 2 parity blocks, so 2 disks can fail. But the parity of RAID 6 isn't dedicated to certain disks, rather it is distributed on all of the. With RAID DP, the parity blocks are on two disks dedicated for parity. For that reason you still had 2 parity disks running OK when you had the issue, the 2 disks that failed happened to be disks with data, and not those dedicated to parity.

But again as has been mentioned already, if a 3rd disk had failed, you'd have lost the data.
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LVL 37

Expert Comment

by:Neil Russell
ID: 40417060
Typo rindi "As stated already it is RAID DP, which is a special form or RAID 6"

should have said "As stated already it is RAID DP, which is a special form OF RAID 6"
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Expert Comment

by:rindi
ID: 40417084
Of course, I didn't notice... I tend have many typo's.
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Expert Comment

by:Neil Russell
ID: 40417085
Really? I never make Mistikes when i type ;)
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LVL 47

Expert Comment

by:dlethe
ID: 40417094
technically you COULD have had data loss with even a single HDD failure, but the odds are quite small.   HDD parity protection data loss scenarios are based on the premise that 100% of the blocks on surviving drives are still readable and the parity is consistent and current.

To make it easy, let's say you just have 2 disks in a RAID1 (mirrored) config.    Block #3 just failed in disk A,  so it is unreadable.  Disk B just died.    You have partial data loss on disk A.  The data on Block#3 is unavailable.

See?  Even RAID DP, or RAID6, and RAIDZ2 levels dont protect against data loss if you have bad blocks at the wrong places when disks survive.
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Accepted Solution

by:
andyalder earned 500 total points
ID: 40417195
Please read the question again gentlemen, it says "How many drives can fail in a RAID-DP netapp filer **aggregate**", not how many can fail in a RAID-DP disk group. If there are 5 disk groups in the aggregate then 10 drives could fail before data loss if you were very lucky and lost two from each group and didn't have any bad blocks.
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Author Comment

by:philb19
ID: 40417241
Thats A great answer     I couldnt tell what raid group they had come from.      Sysconfig -r. Just said broken 2 disks
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