VCAP-DCA Objective 1.2 – Manage Storage Capacity in a vSphere Environment

  • Identify storage provisioning methods
  • Identify available storage monitoring tools, metrics and alarms
Skills and Abilities
  • Apply space utilization data to manage storage resources
  • Provision and manage storage resources according to Virtual Machine requirements
  • Understand interactions between virtual storage provisioning and physical storage provisioning
  • Apply VMware storage best practices
  • Configure datastore alarms
  • Analyze datastore alarms and errors to determine space availability
  • vSphere Datacenter Administration Guide
  • Fibre Channel SAN Configuration Guide
  • iSCSI SAN Configuration Guide
  • vSphere Command-Line Interface Installation and Scripting Guide
  • Product Documentation
  • vSphere Client
  • vSphere CLI
    • vmkfstools

Identify storage provisioning methods

There are two methods for provisioning storage.

  • Thin provisioning
  • Thick (FAT) provisioning

What is a thin provisioned disk?

When creating a virtual disk file, by default, VMware ESX uses a thick type of virtual disk. The thick disk pre-allocates all of the space specified during the creation of the disk. For example, if you create a 10 megabyte disk, all 10 megabytes are pre-allocated for that virtual disk. In contrast, a thin virtual disk does not pre-allocate all of the space. Blocks in the VMDK file are not allocated and backed by physical storage until they are written during the normal course of business. A read to an unallocated block returns zeroes, but not back the block with physical storage until it is written. See VMware KB 1005418


The following are some considerations when implementing thin provisioning in your VMware environment:

  • Thin provisioned disks can grow to the full size specified at the time of virtual disk creation, but do not shrink. Once the blocks have been allocated, they cannot be un-allocated.
  • By implementing thin provisioned disks, you are able to over-allocate storage. If storage is over allocated, thin virtual disks can grow to fill an entire datastore if left unchecked.
  • VMware ESX 3.x is not aware of thin provisioning when reporting disk space usage using VMware Infrastructure Client and VirtualCenter.
  • VMware ESX 4.x is aware of thin provisioning in the form of the storage views plugin for vCenter.
  • When a thin provisioned disk grows, the VMFS metadata must be locked to order to make changes to a file. The VMware ESX host must make a SCSI reservation to make this changes. For more information about SCSI reservations, see Analysing SCSI Reservation conflicts on VMware Infrastructure 3.x and vSphere 4.x (1005009).
  • In order for a guest operating system to make use of a virtual disk, the guest operating system must first partition and format the disk to a file system it can recognize. Depending on the type of format selected within the guest operating system, the format may cause the thin provisioned disk to grow to a full size.

For example, if you present a thin provisioned disk to a Microsoft Windows operating system and format the disk, unless you explicitly select the Quick Format option, the Microsoft Windows format tool writes information to all of the sectors on the disk, which in turn inflates the thin provisioned disk.

The performance of a thin disk is the same as a Thick disk, that is what the document called Performance Study of VMware vStorage Thin Provisioning is saying see



Identify available storage monitoring tools, metrics and alarms

Storage can be monitored with a few tools from VMware, there are also a few nice ones from 3rd party software companies like Veaam or Vizoncore.

The VMware tools are:

  • vscsiStats
  • VMware vCenter
  • VMware vSphere Client.

Eric Siebert has written an article about VMware vSphere’s built-in performance monitoring tools, see his article at

VMware Knowledgebase article 1008205 describes how to use ESXTOP and or RESXTOP to identify performance issues. See

For some more info about the vscsiStats see Yellow Bricks article

There is also a special VMware training available, this is called: VMware vSphere: Skills for Storage Administrators. See Course datasheet at

Scott Sauer has created an article about Performance Troubleshooting on Storage, see He also discusses some tools used on different levels in the VMware infrastructure.


Apply space utilization data to manage storage resources

In general you never want to have less than 20% space free. Other than that studying should be focused around on how to check these statistics out. Source Sean Crookston VCPA-DCA Study guide.

How to check these statistics, see the other study objects in this chapter. Some methods are setting alarms in vCenter on the Datastores.

Jeremy Waldrop has an blog article written about the new alarms that are available in vCenter 4. See


Provision and manage storage resources according to Virtual Machine requirements

The blog did an article about Virtual Machine Storage Provisioning and best practices. See

VMware has also created an document called StorageWorkload Characterization and Consolidation in Virtualized Environments. You can find it here:

Microsoft has a document to analyse storage performance, see

Understand interactions between virtual storage provisioning and physical storage provisioning


When you thin provision virtual machines you must account for the possibility of these virtual machines growing. It is often common nowadays to overprovision storage with thin provisioning and the risk is there that you could run out of physical storage as result. This is a very good use case for alarms in vCenter.

Additionally the physical storage provisioned will affect the performance of the guest. Read the other topics on storage already covered in object 1.1 to understand the different raid levels and how they can affect performance.

Apply VMware storage best practices


Many of the best practices for physical storage environments also apply to virtual storage environments. It is best to keep in mind the following rules of thumb when configuring your virtual storage infrastructure:


Configure and size storage resources for optimal I/O performance first, then for storage capacity.

This means that you should consider throughput capability and not just capacity. Imagine a very large parking lot with only one lane of traffic for an exit. Regardless of capacity, throughput is affected. It’s critical to take into consideration the size and storage resources necessary to handle your volume of traffic—as well as the total capacity.

Aggregate application I/O requirements for the environment and size them accordingly.

As you consolidate multiple workloads onto a set of ESX servers that have a shared pool of storage, don’t exceed the total throughput capacity of that storage resource. Looking at the throughput characterization of physical environment prior to virtualization can help you predict what throughput each workload will generate in the virtual environment.

Base your storage choices on your I/O workload.

Use an aggregation of the measured workload to determine what protocol, redundancy protection and array features to use, rather than using an estimate. The best results come from measuring your applications I/O throughput and capacity for a period of several days prior to moving them to a virtualized environment.

Remember that pooling storage resources increases utilization and simplifies management, but can lead to contention.

There are significant benefits to pooling storage resources, including increased storage resource utilization and ease of management. However, at times, heavy workloads can have an impact on performance. It’s a good idea to use a shared VMFS volume for most virtual disks, but consider placing heavy I/O virtual disks on a dedicated VMFS volume or an RDM to reduce the effects of contention.

VMware document: Dynamic Storage Provisioning. Considerations and Best Practices for Using

Virtual Disk Thin Provisioning

VMware document: Best Practices for Running vSphere on NFS Storage



Configure datastore alarms

See the vSphere Datacenter administration guide chapter 13, Working with Alarms.

Jeremy Waldrop has an blog article written about the new alarms that are available in vCenter 4. See



Analyse datastore alarms and errors to determine space availability

There are different posts in the VMware community site of VMware. See the following.



Documents and manuals

Vsphere Datacenter administration Guide:

Fibre Channel SAN Configuration Guide:

iSCSI SAN Configuration Guide:

vSphere Command-Line Interface Installation and Scripting Guide:



If there are things missing or incorrect please let me know.

Related articles:

The information in this article is provided “AS IS” with no warranties, and confers no rights. This article does not represent the thoughts, intentions, plans or strategies of my employer. It is solely my opinion.


Marco works for ViaData as a Senior Technical Consultant. He has over 15 years experience as a system engineer and consultant, specialized in virtualization. VMware VCP4, VCP5-DC & VCP5-DT. VMware vExpert 2013, 2014,2015 & 2016. Microsoft MCSE & MCITP Enterprise Administrator. Veeam VMSP, VMTSP & VMCE.