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Raid 5 Explanation

Posted on 2006-11-17
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Last Modified: 2006-12-11
I am currently running Raid-5 in a 3 disk scenario.  My Dell Server has 6 disk drive slots.

I understand if one disk fails, the other two will continue to function fine.

I will be adding 3 addtional drives for a total of 6.

Which of the following two assumptions is correct:

Assumption A:

In a 6 disk (Raid-5) scenario, if one drive is lost, the other five will function fine.  The Server would then "rebuild" redundancy in a 5 disk scenario.

If 2 drives fail, 4 would run fine under Raid-5
If 3 driver fail, 3 would run fine under Raid-5
If 4 drives fail, 2 would run fine under Raid-5
If 5 drives were lost, all data would be lost

Assumption B:

If any two of the six drives fail, all data would be lost.

Thanks for your help.
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Question by:jimdorman
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nitadmin earned 175 total points
ID: 17970322
Hi jimdorman,

Assumption A is wrong.
Assumption B is True.

When you create a Raid 5 volume, you need minimum 3 physical drives. If you have more than 3 drives. You not increasing you redundancy, and you are not exapnding you drive space. And you definately not increasing your read performance. The only thing that you are increasing is the costs.

When one of of your RAID 5 Drives fail, then yes you will not lose data, but your server will not function as if nothing has happened. You still need to replace the failed hard drive in order to rebuild your raid 5 volume. This is true no matter how many physical disk drives you have in your RAID 5 volume. Your assumption that if you have 6 drives and one fails, you can continue to run the server without replacing your failed drive is wrong. The failed drive needs to be replaced for your server to function properly. However, you will not lose data. If multiple drives in your Raid 5 volume fail at the same time. Then yes, you will lose data. Also lets say you have one RAID 5 controller and all the Disk drives are attached to this RAID controller. If the RAID controller fails, your server will go down. This is why you should not have a single raid controller for all your physical hard drives. Ideally, you should have a separate Raid controller for each physical hard drive.

Cheers!
NITADMIN
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by:elbereth21
elbereth21 earned 75 total points
ID: 17971447
To clarify what NITADMIN wrote, RAID 5 always takes away one of the available disks for parity (in other words, it has an overhead of 1/n, where n is the number of disks used), so if you have a RAID 5 based on three disks, you will have an available total space of two disks. If instead, you create a RAID of six disks, than you will see an available total space of five disks.
This also means, as NITADMIN said, that if any disk fails, then the RAID runs in "degraded state", because there is no more redundancy. As soon as another disk fails, you have lost all of your data.
See also here: www.pcguide.com/ref/hdd/perf/raid/levels/index.htm
http://www.acnc.com/04_01_05.html
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