HPE Nimble Storage Solution Specialists

# Usable Capacity calculation

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## Usable Capacity calculation

Hi Team,

if you have twelve 4TB drives installed into an array, the raw space is 48TB (12TB * 4 = 48TB)  In this example, we have 48TB of Raw Capacity. From that we subtract all overhead associated with RAID Triple parity we can see 36TB  but one of the document says that 33TB am not sure howmuch other capacity used for other matics like volume reservations, system spares etc to make 33TB usable?

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## Re: Usable Capacity calculation

You should refering to the old gen adaptive flash CSxxx, which comes with 4 x SSD and 12 x NL-SAS.

4TB drive is marketing figure and in fact it doesn't shiped with 4TB disk blocks.

Therefore, by reducing 3 drives for standard triple parity,  plus system reserve and checksum, it gives 33TB usable.  But with compression is always ON, and based on the common data types in actual deployement we see, effective capacity should always yields more than that.

Thomas Lam - Global Storage Field CTO

I work for HPE
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Solution

## Re: Usable Capacity calculation

Nimble Array Models such as AF20/AF40/AF60/AF80 require minimum 24 drives.
Nimble Array Models such as HF20/HF20C/HF40/HF40C/HF60 require minimum 3 SSD + 21 NL-SAS drives.
Nimble Array HF20H require minimum 1 SSD + 11 NL-SAS drives
Nimble Array AF20Q is the one which can work with only 12 SSD drives.
If you are talking about 12 drives, I guess you must be talking about AF20Q. In AF-series Arrays, we have spare chunks in every drive which will never be used for storing Data or Parity. It is used for storing reconstructed data when a drive fails.
Out of 12 drives, 1 drive worth of space is marked as Spare chunks (Distributed Sparing). So you are left with space equal to 11 drives.
In your example, each drive is 4 TB. So total RAW space available for Data + Parity is 4 X 11 = 44 TB
When you start creating Volumes, Nimble Array Controller will create so many RAID sets with set size (8 + 3).
Now, If you Calculate...For every 8 GB of Data written -> 3 GB of space is required for Parity.
So out of 44 TB of RAW space, how much will be for Data & how much for Parity?
Approx, 32 GB for Data + 12 GB for Parity.

I work for HPE