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.