When reviewing storage data in OneIQ, you may notice that reported capacity and usage values differ across sources. This is expected and reflects how infrastructure platforms calculate and report storage. This article explains the key figures, what they mean, and why they don’t always match.
Types of Storage Figures You May See
Provisioned Capacity
The total virtual disk space allocated to workloads (for example, a VM with a 500 GB VMDK).
Includes allocated space even if it is not yet written to.
Common in hypervisor-level views.
Used Capacity (Guest OS)
What the operating system inside the VM reports as used.
Reflects files, applications, and OS usage.
Does not always align with the provisioned size due to thin provisioning.
Datastore / Array Reported Usage
What the storage system itself reports at the volume or datastore level.
Can include metadata, snapshots, replication, and efficiency overheads.
This is often higher than guest-level reporting.
Effective Capacity
Accounts for deduplication, compression, and erasure coding.
Reflects usable space after storage efficiencies.
It may differ significantly from raw physical capacity.
Raw Capacity
The physical total before RAID, mirroring, or erasure coding.
Rarely matches what is usable.
Why the Numbers Differ
Hypervisor vs Guest OS
VMware or Hyper-V reports what the VM could use (provisioned) while the OS shows what is actually in use.
Snapshots and Clones
These consume datastore space but are not visible inside the VM.
RAID, RF, and Erasure Coding
RAID and Nutanix RF2/RF3 or VMware vSAN FTT1/FTT2 policies reserve additional capacity for fault tolerance.
Example: RF2 halves usable space because each block is mirrored. RF3 keeps three copies.
vSAN erasure coding (FTT1 with RAID-5/6) reduces efficiency compared to mirroring but saves space.
Deduplication and Compression
Can reduce the effective usage compared to raw usage. Array-level efficiencies are not reflected in VM guest usage.
.VMX and Overheads
VMware creates support files (.vmx, .vswp, logs) that consume space in datastores but are not visible in guest OS reporting.
Block Size and Metadata
Storage arrays reserve space for metadata and padding that increases datastore usage beyond what workloads show.
What’s Included in OneIQ Figures
OneIQ surfaces storage values as reported by the source platform (hypervisor, OS, or array).
Guest OS usage is taken from inside the VM.
Datastore/array usage is taken from the underlying platform APIs (VMware vCenter, Nutanix Prism, etc.).
Figures do not normalize across platforms; they reflect the native platform’s perspective.
Mirroring, erasure coding, and efficiency savings are included only if the platform exposes them. For example, VMware vSAN reports capacity after applying storage policy overheads, while Nutanix Prism shows both raw and effective.
Key Takeaway
Differences between figures are normal because each layer (guest, hypervisor, storage) has its own perspective:
Guest OS: “What my files use”
Hypervisor: “What I’ve allocated to this VM”
Storage system: “What is actually consumed on disk, with all policies and overheads applied”
Understanding which layer a number comes from is essential to interpreting storage correctly in OneIQ.
FAQ
Why is Logical Capacity larger than Provisioned Capacity?
This happens when the guest operating systems have access to storage that is not counted in VMware’s VMDK provisioned totals. Logical shows everything the OS can see, while Provisioned only shows VMDKs allocated by vSphere.
Common reasons include:
Raw Device Mappings (RDMs): The VM is attached directly to a LUN, bypassing VMFS.
In-guest iSCSI or NFS mounts: The OS mounts additional disks directly from a storage array.
Clustered volumes: Shared storage presented at the OS level that vSphere does not account for.
Key point:
Provisioned = VMware’s perspective (VMDKs).
Logical = the guest OS’s perspective (all storage, including RDMs and in-guest mounts).