/vSAN fix for small magnetic disks

vSAN fix for small magnetic disks

If you are using small magnetic disks in your vSAN than, smaller than 255 Gb, then you need a vSAN fix for small magnetic disks.

If you do not change your vSAN settings to allow SSD disks smaller than 255Gb, your virtual machines that run on vSAN datastores might experience disk space issues.

When vSAN stores virtual machine objects, it creates components whose default size does not exceed 255 GB. However, when you use magnetic disks that are smaller than 255 GB, these storage components might end up to be larger than actual underlying magnetic disks. As a result, virtual machines associated with these storage components experience unpredictable behavior and might fail.

If you have SSD smaller than 255Gb you should see a similar warning in your vSAN health monitor:


To avoid this issue, you need to adjust the default value of the ClomMaxComponentSizeGB parameter for each host in the vSAN cluster.

Note: In this case is my Nested vSAN that only have 20Gb SSD disks. Is not the best example since no one should build a vSAN with 20Gb. But this environment is just for testing purposes, and since I am using smaller disks, I get the warning. But in a real production environment, this issue could corrupt your Virtual Machines.

Wow to fix it.

  • Select ESXi host and click Configure TAB.
  • Under System, click Advanced System Settings and click Edit.
  • In the filter search for ClomMaxComponentSizeGB.
  • Change the value with the proper value for your environment.


In my case, I changed the ClomMaxComponentSizeGB to the size my vSAN health monitor recommended 180Gb.

You can use the recommended size (because vSAN will check your environment and propose a proper size).

After setting the value to 180Gb in each ESXi host and click “Retest with Online Health” the warning disappear, and all is green now.


Changing this setting should be done before you deploy any VMs in your vSAN Datastores. When you already have deployed VMs in your vSAN environment, then you should do some changes at Storage Policy level so that Object Space Reservation is adequately set for those VMs.

You can preserve object space for the previously deployed objects by applying a storage policy with Object Space Reservation is set to 100% with these considerations:
  • For Virtual Machines with single VMDKs, apply the policy to one virtual machine at a time
  • For Virtual Machines with multiple VMDKs, apply the policy to one VMDK at a time

For more information and calculations for ClomMaxComponentSizeGB check VMware KB regarding this subject.

Hope this tip could help you improving your vSAN environment in this TIP vSAN series that I will write.

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By | 2018-09-20T18:12:23+02:00 April 27th, 2018|Storage, VMware Posts, vSAN|0 Comments

About the Author:

I am over 20 years’ experience in the IT industry. Working with Virtualization for more than 10 years (mainly VMware). I am an MCP, VCP6.5-DCV, VMware vSAN Specialist, Veeam Vanguard 2018/2019, vExpert vSAN 2018/2019 and vExpert for the last 4 years. Specialties are Virtualization, Storage, and Virtual Backups. I am working for Elits a Swedish consulting company and allocated to a Swedish multinational networking and telecommunications company as a Teach Lead and acting as a Senior ICT Infrastructure Engineer. I am a blogger and owner of the blog ProVirtualzone.com

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