Reserving PVC space for file system overhead

When you add a virtual machine disk to a persistent volume claim (PVC) that uses the Filesystem volume mode, you must ensure that there is enough space on the PVC for the VM disk and for file system overhead, such as metadata.

By default, OpenShift Virtualization reserves 5.5% of the PVC space for overhead, reducing the space available for virtual machine disks by that amount.

You can configure a different overhead value by editing the HCO object. You can change the value globally and you can specify values for specific storage classes.

Overriding the default file system overhead value

Change the amount of persistent volume claim (PVC) space that the OpenShift Virtualization reserves for file system overhead by editing the spec.filesystemOverhead attribute of the HCO object.

Prerequisites

Install the OpenShift CLI (oc).

Procedure

Open the HCO object for editing by running the following command:

$ oc edit hyperconverged kubevirt-hyperconverged -n openshift-cnv

Edit the spec.filesystemOverhead fields, populating them with your chosen values:
```
# ...
spec:
  filesystemOverhead:
    global: "<new_global_value>"
    storageClass:
      <storage_class_name>: "<new_value_for_this_storage_class>"
```
- spec.filesystemOverhead.global specifies the default file system overhead percentage used for any storage classes that do not already have a set value. For example, global: "0.07" reserves 7% of the PVC for file system overhead.
- spec.filesystemOverhead.storageClass specifies the file system overhead percentage for the specified storage class. For example, mystorageclass: "0.04" changes the default overhead value for PVCs in the mystorageclass storage class to 4%.
Save and exit the editor to update the HCO object.

Verification

View the CDIConfig status and verify your changes by running one of the following commands:

To generally verify changes to CDIConfig:
```
$ oc get cdiconfig -o yaml
```
To view your specific changes to CDIConfig:
```
$ oc get cdiconfig -o jsonpath='{.items..status.filesystemOverhead}'
```