Ondat Volumes


Ondat volumes are a logical construct which represent a writeable volume and exhibit standard POSIX semantics. Ondat presents volumes as mounts into containers via the Linux-IO (LIO) subsystem.

Conceptually, Ondat volumes have a frontend presentation, which is what the application sees, and a backend presentation, which is the actual on-disk format. Depending on the configuration, frontend and backend components may be on the same or different hosts.

Volumes are formatted using the linux standard ext4 filesystem by default. Kubernetes users may change the default filesystem type to ext2, ext3, ext4, or xfs by setting the fsType parameter in their StorageClass - review the Supported Filesystems page for more information.

💡 Different filesystems may be supported in the future.

Ondat volumes are represented on disk in two parts.

  • Actual volume data is written to blob files in /var/lib/storageos/data/dev[\d+]. Inside these directories, each Ondat block device gets two blob files of the form vol.xxxxxx.y.blob, where x is the inode number for the device, and y is an index between 0 and 1.
  • We provide two blob files in order to ensure that certain operations which require locking do not impede in-flight writes to the volume.

In systems which have multiple /var/lib/storageos/data/dev[\d+] directories, two blob files are created per block device.

  • This allows us to load-balance writes across multiple devices. In cases where dev directories are added after a period of runtime, later directories are favoured for writes until the data is distributed evenly across the blob files.

Metadata is kept in directories named /var/lib/storageos/data/db[\d+].

  • We maintain an index of all blocks written to the blob file inside the metadata store, including checksums. These checksums allow us to detect bit rot, and return errors on reads, rather than serve bad data.
  • In future versions we may implement recovery from replicas for volumes with one or more replicas defined.
  • Ondat metadata requires approximately 2.7 GiB of storage per 1 TiB of allocated blocks in the associated volume. This size is consistent irrespective of data compression defined on the volume.

To ensure deterministic performance, individual Ondat volumes must fit on a single node.

Minimum Ondat Volume Size

  • The minimum volume size Ondat supports is 1 GiB.

Ondat Volume Resizing

Ondat supports online and offline resizing of volumes.


  • This means that a volume cannot be resized while it is in use. Furthermore, in order for a resize operation to take place the volume must not be attached to a node. This is to ensure that the volume is not in use.
  • This means that if a Kubernetes pod is currently consuming a volume that a resize request has been issued for, the resize will not be actioned until the pod is terminated and the volume is detached from the node.
  • The Ondat Control Plane will then attach the volume to the node that holds the master deployment and resize the underlying block device and then run resize2fs or xfs_growfs, on ext4 and XFS respectively, to expand the filesystem.


This feature is available in release v2.9.0 or greater.

  • This means that volumes can be resized when in use or when not in use.
  • The Ondat Control Plane will resize the underlying block device and run resize2fs or xfs_growfs, on ext4 and XFS respectively, to expand the filesystem whilst allowing the volume to continue to be used.

For more information on how to resize a volume, review the Volume Resize operations page.

Ondat Volume Encryption

Volumes can be configured on creation to have data encryption-at-rest. Data is encrypted with XTS-AES and decrypted upon use.

For more information on how to enable data encryption for Ondat volumes, review the Ondat Data Encryption feature page.


💡 This feature is available in release v2.4.0 or greater.

Ondat volumes support TRIM/UNMAP which allows the space allocated to deleted blocks to be reclaimed from the backend blob files that back each volume when a TRIM call is made.

  • Support for TRIM is enabled by default for all uncompressed volumes, volumes are created without compression enabled by default.

For more information on how to TRIM a filesystem, review the TRIM operations page.