Lazy Zero Thick vs Eager Zero Thick vs Thin Provisioned Virtual Disk



Most Frequently asked questions by VMware Administrators/Support Engineers

  • What is the difference between Eager Zeroed and Lazy Zeroed Disk?
  • What kind of disk should be selected “Thin Disk or Thick Disk” During VM deployment?
  • Is there any performance difference between Thin and Thick Virtual Disk?
  • Is there any potential performance difference between Lazy Zero and Eager Zero Thick Virtual Disk?
  • What type of virtual disk is recommended in customer environment?

If such queries had bothered you, i would say you are at the right place & I promise it won’t trouble you any longer.
In this article, i am going to address all these queries with the help of customer case study so stay tuned.

Let’s get Started but before digging into how different disk type affect Virtual Machine Write Performance, let’s first understand the difference between Thin and Thick Provisioning Virtual Disk. 


Thin Provisioning is VMware vSphere Technology allows Virtual Disk storage allocation on demand. In other words, allocate storage space only when VM needs it.
In thin provisioning, all of the VM’s configured storage doesn’t get allocated to VM during creation. Instead, space allocation happens as when VM virtual disk starts growing.

Advantage of Thin Provisioning

  • Thin Provisioning cut down storage costs
  • Efficient use of Storage Space

Disadvantage of Thin Provisioning

  • Overallocation of storage might cause Datastore Outage
  • Performance Overhead
  • Manageability Overhead from Admin Perspective

How to Create Thin Provision Virtual DiskThinProvisioningCreationScreenshot

  1. Create New Virtual Machine using New Virtual Machine Wizard or Add New Disk via Virtual Machine Edit Settings
  2.  In Create Disk Dialogue Box, Select Thin Provision and Click Next.
  3. Virtual Disk in Thin Format will be created.


In simple words, Thick Provisioning means allocating all storage space to VM from underlying VMFS volume at the time of creation itself. In other words, Allocate entire configured space to Virtual Disk during VM provisioning.

Advantage of Thick Provisioning

  • No Performance Overhead
  • Easy to Manage
  • Less prone to Storage Outage because no overcommitment of Storage

Disadvantage of Thick Provisioning

  • In-efficient use of Storage Resources
  • Waste of Storage Space
  • Expensive approach in terms of costs

How to Create Thick Provisioning Virtual DiskThickProvisioningCreationScreenshot

  1. Create New Virtual Machine using New Virtual Machine Wizard or Add New Disk via Virtual Machine Edit Settings.
  2. In Create Disk Dialogue Box, Select Thick Provision lazy zero or Eager Zero and Click Next.
  3. Virtual Disk in Thin Format will be created.

Case Study of Thin and Thick Disk Provisioning

Scenario Details:

  • Two Vms (VM1 and VM2) running on ESXi host.
  • VM1 is configured with 40GB Thick provisioning Eager Zero Disk
  • VM2 is configured with 40GB Thin Provisioning Disk
  • Underlying VMFS Volume Capacity is 80GB


As you can see in above diagram,

For VM1, Entire 40GB is allocated from underlying VMFS volume even though effective storage used by VM is 20GB due to Thick Provisioning configuration.

For VM2, only 20GB storage will be allocated from underlying VMFS volume even though VM is configured with 40GB Storage due to Thin Provisioning Configuration. As when disk requires more space, VM2 disk can grow upto 40GB which is provisioned or max. configured disk capacity for this VM.

Screenshot of VM configured with THIN PROVISIONING

In Above diagram, if you see Win7-VM provisioned storage or configured disk capacity is 36.19GB but currently VM is using only 12.81GB that’s the reason used storage or effective storage is 12.81GB.

Screenshot of VM configured with THICK PROVISIONING


In Above diagram, if you see RHEL VM effectively used storage is same as provisioned storage or configured disk capacity 102GB regardless of how much storage is actually being used by VM.

In Conclusion Thick Provisioning is more static in nature & Thin Provisioning is more dynamic in nature w.r.t. Storage allocation to VM Virtual Disk from underlying VMFS volume.

Thin Provisioning provides Efficient use of storage by providing Overallocation of storage where Thick provisioning provides less overhead and better performance w.r.t. first write operation performed on configured disk.

I am hoping, by now you guys will be clear about Thick and Think Disk Provisioning & how these disk types are different from each other in terms of functionality.

In my next article, we will be digging more into how these three disk types affect virtual machine performance and what are the things you need to take care of while selecting virtual disk during VM creation.

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VMware Solution Architect
I am VMware Solution Architect with 10+ Years of enriching experience in Datacenter Virtualization Technologies, Storage Area Networks and Software Defined Datacenter, Networking and Storage.
I hold Numerous certification including RHCE, CCNA, VCP4.0, VCP5.1, VCP5.5, vCloud and EMC certification.
While spending countless hours exploring the product inside and out and learning everything about it, Eventually I discovered my passion for teaching and helping others learn from my knowledge and experience so turned to Trainer cum Blogger to educate every single person keen to learn Virtualization.

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