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Using preallocation for data volumes

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

You can enable preallocation 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 for a data volume

You can enable preallocation 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 CLI (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.