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CephFilesystem CRD

Rook allows creation and customization of shared filesystems through the custom resource definitions (CRDs). The following settings are available for Ceph filesystems.




This sample requires at least 1 OSD per node, with each OSD located on 3 different nodes.

Each OSD must be located on a different node, because both of the defined pools set the failureDomain to host and the replicated.size to 3.

The failureDomain can also be set to another location type (e.g. rack), if it has been added as a location in the Storage Selection Settings.

kind: CephFilesystem
  name: myfs
  namespace: rook-ceph
    failureDomain: host
      size: 3
    - name: replicated
      failureDomain: host
        size: 3
  preserveFilesystemOnDelete: true
    activeCount: 1
    activeStandby: true
    # A key/value list of annotations
    #  key: value
    #  nodeAffinity:
    #    requiredDuringSchedulingIgnoredDuringExecution:
    #      nodeSelectorTerms:
    #      - matchExpressions:
    #        - key: role
    #          operator: In
    #          values:
    #          - mds-node
    #  tolerations:
    #  - key: mds-node
    #    operator: Exists
    #  podAffinity:
    #  podAntiAffinity:
    #  topologySpreadConstraints:
    #  limits:
    #    memory: "1024Mi"
    #  requests:
    #    cpu: "500m"
    #    memory: "1024Mi"

(These definitions can also be found in the filesystem.yaml file)

Erasure Coded

Erasure coded pools require the OSDs to use bluestore for the configured storeType. Additionally, erasure coded pools can only be used with dataPools. The metadataPool must use a replicated pool.


This sample requires at least 3 bluestore OSDs, with each OSD located on a different node.

The OSDs must be located on different nodes, because the failureDomain will be set to host by default, and the erasureCoded chunk settings require at least 3 different OSDs (2 dataChunks + 1 codingChunks).

kind: CephFilesystem
  name: myfs-ec
  namespace: rook-ceph
      size: 3
    - name: default
        size: 3
    - name: erasurecoded
        dataChunks: 2
        codingChunks: 1
    activeCount: 1
    activeStandby: true

IMPORTANT: For erasure coded pools, we have to create a replicated pool as the default data pool and an erasure-coded pool as a secondary pool.

(These definitions can also be found in the filesystem-ec.yaml file. Also see an example in the storageclass-ec.yaml for how to configure the volume.)

Filesystem Settings


  • name: The name of the filesystem to create, which will be reflected in the pool and other resource names.
  • namespace: The namespace of the Rook cluster where the filesystem is created.


The pools allow all of the settings defined in the Pool CRD spec. For more details, see the Pool CRD settings. In the example above, there must be at least three hosts (size 3) and at least eight devices (6 data + 2 coding chunks) in the cluster.

  • metadataPool: The settings used to create the filesystem metadata pool. Must use replication.
  • dataPools: The settings to create the filesystem data pools. Optionally (and we highly recommend), a pool name can be specified with the name field to override the default generated name; see more below. If multiple pools are specified, Rook will add the pools to the filesystem. Assigning users or files to a pool is left as an exercise for the reader with the CephFS documentation. The data pools can use replication or erasure coding. If erasure coding pools are specified, the cluster must be running with bluestore enabled on the OSDs.
    • name: (optional, and highly recommended) Override the default generated name of the pool. The final pool name will consist of the filesystem name and pool name, e.g., <fsName>-<poolName>. We highly recommend to specify name to prevent issues that can arise from modifying the spec in a way that causes Rook to lose the original pool ordering.
  • preserveFilesystemOnDelete: If it is set to 'true' the filesystem will remain when the CephFilesystem resource is deleted. This is a security measure to avoid loss of data if the CephFilesystem resource is deleted accidentally. The default value is 'false'. This option replaces preservePoolsOnDelete which should no longer be set.
  • (deprecated) preservePoolsOnDelete: This option is replaced by the above preserveFilesystemOnDelete. For backwards compatibility and upgradeability, if this is set to 'true', Rook will treat preserveFilesystemOnDelete as being set to 'true'.

Metadata Server Settings

The metadata server settings correspond to the MDS daemon settings.

  • activeCount: The number of active MDS instances. As load increases, CephFS will automatically partition the filesystem across the MDS instances. Rook will create double the number of MDS instances as requested by the active count. The extra instances will be in standby mode for failover.
  • activeStandby: If true, the extra MDS instances will be in active standby mode and will keep a warm cache of the filesystem metadata for faster failover. The instances will be assigned by CephFS in failover pairs. If false, the extra MDS instances will all be on passive standby mode and will not maintain a warm cache of the metadata.
  • mirroring: Sets up mirroring of the filesystem
    • enabled: whether mirroring is enabled on that filesystem (default: false)
    • peers: to configure mirroring peers
      • secretNames: a list of peers to connect to. Currently (Ceph Pacific release) only a single peer is supported where a peer represents a Ceph cluster.
    • snapshotSchedules: schedule(s) snapshot.One or more schedules are supported.
      • path: filesystem source path to take the snapshot on
      • interval: frequency of the snapshots. The interval can be specified in days, hours, or minutes using d, h, m suffix respectively.
      • startTime: optional, determines at what time the snapshot process starts, specified using the ISO 8601 time format.
    • snapshotRetention: allow to manage retention policies:
      • path: filesystem source path to apply the retention on
      • duration:
  • annotations: Key value pair list of annotations to add.
  • labels: Key value pair list of labels to add.
  • placement: The mds pods can be given standard Kubernetes placement restrictions with nodeAffinity, tolerations, podAffinity, and podAntiAffinity similar to placement defined for daemons configured by the cluster CRD.
  • resources: Set resource requests/limits for the Filesystem MDS Pod(s), see MDS Resources Configuration Settings
  • priorityClassName: Set priority class name for the Filesystem MDS Pod(s)
  • startupProbe : Disable, or override timing and threshold values of the Filesystem MDS startup probe
  • livenessProbe : Disable, or override timing and threshold values of the Filesystem MDS livenessProbe.

MDS Resources Configuration Settings

The format of the resource requests/limits structure is the same as described in the Ceph Cluster CRD documentation.

If the memory resource limit is declared Rook will automatically set the MDS configuration mds_cache_memory_limit. The configuration value is calculated with the aim that the actual MDS memory consumption remains consistent with the MDS pods' resource declaration.

In order to provide the best possible experience running Ceph in containers, Rook internally recommends the memory for MDS daemons to be at least 4096MB. If a user configures a limit or request value that is too low, Rook will still run the pod(s) and print a warning to the operator log.