Calculating usable disk space

calculating usable disk space you use the physical and usable capacity of the disks you employ in your storage systems to ensure that your storage architecture conforms to the overall system capacity limits and the size limits of your aggregates.

To maintain compatibility across different brands of disks, Data ONTAP rounds down (right-sizes) the amount of space available for user data. In addition, the numerical base used to calculate capacity (base 2 or base 10) also impacts sizing information. For these reasons, it is important to use the correct size measurement, depending on the task you want to accomplish:

  • For calculating overall system capacity, you use the physical capacity of the disk, and count every disk that is owned by the storage system.
  • For calculating how many disks you can put into an aggregate before you exceed its maximum size, you use the right-sized, or usable capacity of all data disks in that aggregate.

    Parity and dparity disks are not counted against the maximum aggregate size.

Using Disk Space Calculator for calculating usable disk space

You can also calculate your raw and usable disk space using this software. Download Software

Also this includes a new feature called Raid Group Size Estimator where you key in disks values, raid type and disk type – the software will attempt to provide the best RG size values either based on NetApp recommendations or Optimal Capacity.(Please note: The Raid group size input in the calculator is only for disk space calculations and this is ignored for the raid group estimator)

I have developed this Raid Group Size Estimator based on many users request. As usual please provide feedback if you do some testing.

Screenshot of new version 2.1 (Software zip attached to this post)

Disk space calculator


A Deep explanation about Usable space and RAW disk space


Calculating the disk space always we confuse, RAW disk space and usable disk space. RAW disk space means directly calculate Disk size multiplied by number of disks. Usable disk space is different than RAW, We have to remove the values of RAID space, Parity bit space and System Usage. We may confuse and we may not get exact value by using above mentioned application will give exact usable space.

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calculating usable disk space

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Ankam Ravi Kumar

Working as Linux / Storage Administrator L3. Interested in sharing the knowledge.