FlashArray capacity values and provisioning (example VMware/VMFS) quickly explained - Basic overview

[ NOTE: machine translation with the help of DeepL translator without additional proofreading and spell checking ]

Since there is always confusion among customers on the topic of capacity reporting/utilization - for storage devices in general with built-in deduplication/compression mechanisms - I decided to briefly explain the key individual values in this short blog.


First, the individual values for the capacity must be explained briefly (in the following examples, a FlashArray with a DataPack of 22TB (10x 2.2TB DFMs) is referenced):

Purity (Label) RAW: is the capacity that the FlashArray physically provides.

-> a 22TB DataPack consisting of 10x 2.2TB DirectFlash modules and physically provides 21.08TB.

FlashArray Usable: is the capacity that the FlashArray physically provides minus the system offset/RAID (the RAIDset is fixed and cannot be freely selected).

-> a 22TB DataPack provides 12.65TB usable. The system therefore needs approx. 8TB as system offsets. This "usable" capacity is the capacity that is actually writable with a data reduction of 1:1.

FlashArray Effective: is the capacity that the FlashArray effectively provides after applying the achieved data reduction (=DR - deduplication, compression).

-> a 22TB DataPack provides 12.65TB of usable capacity. Multiplying this capacity by the achieved data reduction value gives the capacity that can effectively be stored on the FlashArray.

Provisioned Size: is the "virtual" capacity allocated to a host volume.

Virtual Host Written: is the capacity that a host has written to the provisioned host volume, i.e., the volume capacity as perceived by the host without data deduplication/compression.

How is the "Virtual Host Written" capacity calculated?:

Purity Volume Size in P/T/G * (1 - Thin Provisioning Factor) = Virtual Host Written

-> 100G Volume * (1 - 0.8) = 20G (it may be that from the host point of view the used capacity does not match if an UNMAP/Trim was not executed).

Array Host Written: is the capacity written by a host to the provisioned host volume minus the data reduction achieved, i.e. the capacity effectively occupied on the FlashArray.

-> the 100G volume occupies (after thin provisioning) 20G and we achieve a DR of 5:1, so the Array Host Written capacity is 20G divided by 5 4G.

HINT: Purity always reports capacities in TiB = TebiByte in the GUI/CLI.

(the screenshot shows a FlashArray with a 11TB DataPack).


The example system is equipped with 1x 22TB DataPack. This results in a "FlashArray Usable" capacity of approx. 12.65TB / 11.5TiB (without compression, deduplication and thin provisioning). After assuming data compression with a data reduction factor of 5:1, this configuration gives us a "FlashArray Effective" capacity of 63.25TB, which corresponds to 57.5 TiB.

We assume that multiple volumes are provisioned (Provisioned Size) on the FlashArray to an ESX farm totaling 30TB volumes. These will be used as VMFS datastores. Of the provisioned capacity, 28TB are used by various guest operating systems (VMs/Linux, Windows, ...) - this corresponds to the "Virtual Host Written" capacity.

Since our FlashArray achieves a global data reduction of 5:1, after deduplication and compression, only 5.6TB of the maximum available 12.65TB is effectively occupied as "Array Host Written" capacity of the 28TB.

We have received modified requirements:

Volumes are increased from 30TB to 55TB provisioned. New applications are deployed and 45TB of "Virtual Host Written" is consumed instead of 28TB. The DR remains unchanged at 5:1.

Now 9TB "Array Host Written" are occupied.

Let us now take a brief look at a single volume.

A 10TB volume is provisioned. Of the provisioned capacity, 5TB is used by various guest operating systems (VMs/Linux, Windows, ...) - this corresponds to the "Virtual Host Written" capacity.

Since our volume achieves a data reduction of 5:1, after deduplication and compression only 1TB of the 5TB is effectively occupied as "Array Host Written" capacity out of the maximum 12.65TB available.

(For simplicity, the Data Reduction has been kept constant in the examples).

Enjoy Your Pure! :)

More info - Links

All officially published setting options in the GUI but also CLI can be read via the "on-board" user guides of the Pure Storage systems.

Click on "Help" in the Purity main menu.

The User Guide is structured like the main menu and can be opened downwards. A search function is also integrated - within here you can also search for keywords.

WEB: Pure Storage (Pure1) support portal - Ticket system and support *(requires registered FlashSystems)

PHONE: Pure Storage phone support: GER - (+49) (0)800 7239467; INTERNATIONAL - (+1) 650 7294088

WEB: Pure Storage community

WEB: Pure Storage OFFICIAL blog

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