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Why is Network RAID-5 Overprovisioned?

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Selene1
Occasional Collector

Why is Network RAID-5 Overprovisioned?

Hi Everyone, 

 

I have three nodes in my cluster.  On each node, I've allocated about about 4 TB of space to the VSA.

 

I log into the StoreVirtual Management Console and I click on my cluster.  I see Total Space 12,132.25.  So far so good.

 

So at this point I create my first and only RAID-5 volume.  I would anticipate that the size of the volume, being network RAID-5 would be n-1.  So there would be 33% overhead in a 3 node cluster.  So I would expect that my usable space should be about 8 TB.  However, when I create an 8TB volume, it's overprovisioned.  After checking, it becomes overprovisioned at 6TB--something I would expected with mirroring and a 50% overhead.

 

Why is this happening?  Why am I overprovisioned when I create a volume that is 8TB?

 

Thank you,

2 REPLIES
oikjn
Honored Contributor
Solution

Re: Why is Network RAID-5 Overprovisioned?

because NR5 is not like hardware raid5.  It is similar, but not exact.  I forget the exact way it "works" because it doesn't really work and shouldn't be used, but if I remember correctly, its something like a NR10 volume with some kind of snapshot which simulates a parity drive.  The result is that it can save some space over NR10, but really only with larger clusters, but at a huge performance hit.

 

Short answer is that you should NOT use NR5 unless you are creating a read-only archive for something like image retention or an ISO library.  NR5 has a huge write penalty and it really doesn't work for anything but archives.  My guess from what you are saying is that you are planning on using it for regular production storage and so I'll save you the white hair and tell you now to abort that plan and change the LUNs back to NR10 and either live with the 6TB capacity or add additional raw storage to the nodes.

 

 

Selene1
Occasional Collector

Re: Why is Network RAID-5 Overprovisioned?

Thank you for the input.  I was able to speak to HP support and they also confirmed that network RAID-5 is just not recommended except for the very lightest of loads.  Network RAID-5 doesn't calculate space in a similar method as one would expect, so seeing missing disk space on a cluster of 3 nodes wasn't much of a surprise.

 

I decided to take the cut in disk space and roll forward with network RAID-10.  I wish I had more disk space, but new hard drives can always be purchased down the road.