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Pools and underlying RAID groups size
Just a theoretical question. When a disk pool is created on a VNX block storage, it's based on underlying RAID group technology hidden from the storage administrator. The RAID type is specified during the pool creation, and it can range from 4+1 RAID5 to 14+2 Raid 6 on the current VNX series. In theory, it means that all the disk in the pool are consolidated into corresponding RAID groups invisible to anyone.
However, if we add the number of disks not equal to MxN, where M is the RAID group size specified during creation, the actual size of underlying RAID groups changes. At least, it follows from my experience. If I add 20 disks to a pool based on 8+1 RAID5 groups, its final formatted size is equal to the size of 2 x RAID5 x 10 disks each. If I create the same pool based on only 5 HDDs, it's formatted size is equal to the size of a RAID5 based on 5 disks. And so on. Is this true?
Is there some kind of document (a white paper or something like that) which describes how pools on VNX work? I was unable to locate it by myself.
victory_is_mine
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July 15th, 2014 11:00
I know that I should not. Will the system warn you or prevent you from doing it?
dynamox
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July 15th, 2014 11:00
if i remember correctly it will say that it is not an optimal number of drives but won't stop you (as long as you meet minimum driver number requirement for that raid type)
victory_is_mine
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July 15th, 2014 12:00
Interesting. I will check it out at my next deployment next week, if I do not forget.
lotto741
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July 15th, 2014 21:00
Yes, I trust that's correct. When I was experimenting with differently sized pools with different RAID types (it's always fun playing with a brand-new hardware, you know
), the system allowed me to pick any number of drives for the pool even it wasn't optimal (in its opinion). I believe that it'd demonstrate the same behavior when adding new disks to an existing pool.
Thanks everybody for you input. It was quite an interesting discussion. I wonder if I can ask somebody to create a small FAQ basing on it because this important information is nowhere to be found on the official EMC site.
dynamox
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July 16th, 2014 04:00
yes you can верить мне
. It's always an interesting topic to discuss. There have been a few discussions about this in the past, like this one:
Raid levels in a pool
lotto741
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July 16th, 2014 05:00
Thanks. I'll read that thread as well.