Using preallocation for data volumes - Virtual machines | OpenShift Virtualization

About preallocation
Enabling preallocation globally
Enabling preallocation for a data volume

The Containerized Data Importer can preallocate disk space to improve write performance when creating data volumes.

You can enable preallocation globally for the cluster or for specific data volumes.

About preallocation

The Containerized Data Importer (CDI) can use the QEMU preallocate mode for data volumes to improve write performance. You can use preallocation mode for importing and uploading operations and when creating blank data volumes.

If preallocation is enabled, CDI uses the better preallocation method depending on the underlying file system and device type:

fallocate: If the file system supports it, CDI uses the operating system’s fallocate call to preallocate space by using the posix_fallocate function, which allocates blocks and marks them as uninitialized.
full: If fallocate mode cannot be used, full mode allocates space for the image by writing data to the underlying storage. Depending on the storage location, all the empty allocated space might be zeroed.

Enabling preallocation globally

You can enable cluster-wide preallocation mode for the Containerized Data Importer (CDI) by adding the preallocation field to the CDI object.

Prerequisites

Install the OpenShift client (oc).

Procedure

Use the oc client to edit the CDI object:
```
$ oc edit cdi
```

Set the spec.config.preallocation field with a value of true:

apiVersion: cdi.kubevirt.io/v1beta1
kind: CDI
metadata:
...
spec:
  config:
    preallocation: true (1)

1	The `preallocation` field is a boolean that defaults to false.

Save and exit your editor to update the CDI object and enable preallocation mode.

Enabling preallocation for a data volume

If preallocation is not globally enabled for the cluster, you can enable it for specific data volumes by including the spec.preallocation field in the data volume manifest. You can enable preallocation mode in either the web console or by using the OpenShift client (oc).

Preallocation mode is supported for all CDI source types.

Procedure

Specify the spec.preallocation field in the data volume manifest:
```
apiVersion: cdi.kubevirt.io/v1beta1
kind: DataVolume
metadata:
  name: preallocated-datavolume
spec:
  source: (1)
    ...
  pvc:
    ...
  preallocation: true (2)
```
1 All CDI source types support preallocation, however preallocation is ignored for cloning operations.

2 The preallocation field is a boolean that defaults to false.

1	All CDI source types support preallocation, however preallocation is ignored for cloning operations.
2	The `preallocation` field is a boolean that defaults to false.