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Reserving PVC space for file system overhead

By default, the OpenShift Virtualization reserves space for file system overhead data in persistent volume claims (PVCs) that use the Filesystem volume mode. You can set the percentage to reserve space for this purpose globally and for specific storage classes.

How file system overhead affects space for virtual machine disks

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 virtual machine disk.

  • The space reserved 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

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

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

  2. Edit the spec.filesystemOverhead fields, populating them with your chosen values:

    # ...
spec:
  filesystemOverhead:
    global: "<new_global_value>" (1)
    storageClass:
      <storage_class_name>: "<new_value_for_this_storage_class>" (2)
    1 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.
    2 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%.

  3. 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}'