What is called Disk Provisioning When you create a virtual machine (VM), you also need to create a virtual disk. This disk is where the VM’s operating system, applications, and files are stored. But before creating the disk, you must choose how the storage will be allocated. This is called disk provisioning.
The three most common provisioning types are:
- Thin Provisioning
- Thick Provisioning
- Sparse Disks (mainly in KVM/QCOW2)
1. Thin Provisioning — “Use space only when needed”
Thin Provisioning like booking a large suitcase but filling it only when required.
How it works
You may create a 100 GB disk, but it won’t use the entire 100 GB from your storage immediately.
Instead, it will start small (maybe 5–10 GB) and grow only as the VM stores more data.
Why people use it
- Saves a lot of storage
- Perfect for testing, labs, or cloud environments
- Allows more VMs on the same storage
Things to watch out for
If too many thin disks grow at the same time, your datastore might run out of storage.
So monitoring is important.
2. Thick Provisioning — “Reserve everything from the beginning”
Thick Provisioning is like booking a full 100-GB suitcase and immediately filling it with padding so no one else can use that space.
There are two sub-types:
A. Thick – Lazy Zeroed
“Reserve everything now, clean later.”
How it works
- The full disk size (example: 100 GB) is allocated at creation.
- Data blocks are cleaned (zeroed) only when the VM writes to them for the first time.
Good for
- Medium performance workloads
- General production VMs
Not ideal for
- Workloads with sensitive security needs
- VMs that require top speed on the first write
B. Thick – Eager Zeroed
“Reserve everything and clean everything right now.”
How it works
- Full disk size is allocated immediately
- Every block is cleaned (zeroed) during creation
- Takes the longest time to create but gives the best performance
Best for
- Databases
- High performance workloads
- VMware features like Fault Tolerance (FT)
Drawback
- Slow to create
- Uses full storage instantly
3. Sparse Disks (KVM/QCOW2) — “Thin disk with extra features”
Sparse disks are commonly used in KVM, Proxmox, OpenStack, CloudStack, and mostly based on the QCOW2 format.
What makes sparse disks special
They grow like thin disks, but they also support:
- Snapshots
- Compression
- Encryption
- Copy-on-write (COW)
This makes them more flexible, especially for cloud environments.
Best for
- Cloud templates
- VM snapshots
- Environments where space efficiency matters
Not ideal for
- Heavy I/O workloads (RAW disks are faster)
- High-performance databases