> ## Documentation Index
> Fetch the complete documentation index at: https://private-7c7dfe99-sync-clickhouse-operator-docs-7e82242.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# FAQ

### zone details couldn't be found for any PV?

If this error shows up in the ClickHouse operator logs it means it isn't able to complete reconciliation of the ClickHouse Cluster because the operator expects topology aware node affinities to be automatically populated on the PersistentVolumes. Specifically, the operator looks for node affinities based on the `topology.kubernetes.io/zone` label. Normally this is handled by topology-aware volume provisioners (e.g. when using the AWS EBS volume provisioner).

However, for certain cloud providers (e.g. IBM) or on-premise environments this won't be automatically populated or non-standard labels will be used. To ensure the operator can still continue operations it is recommended to add the node affinities on the storage class, like for example:

```yaml theme={null}
apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: local-nvme-sc
allowVolumeExpansion: true
parameters:
  path: /nvme/disk
provisioner: rancher.io/local-path
reclaimPolicy: Delete
volumeBindingMode: WaitForFirstConsumer
allowedTopologies:
  - matchLabelExpressions:
      - key: topology.kubernetes.io/zone
        values:
          - <your_cluster_name>-keeper
          - <your_host_1>
          - <your_host_2>
          - <your_host_3>
          - <your_host_4>
          - <your_host_5>
          - <your_host_6>
      - key: directpv.min.io/zone
        values:
          - default
```

This will ensure storage volumes are created with the correct node affinities. For example:

```yaml theme={null}
apiVersion: v1
kind: PersistentVolume
metadata:
  ...
spec:
  accessModes:
    - ReadWriteOnce
  ...
  nodeAffinity:
    required:
      nodeSelectorTerms:
        - matchExpressions:
            - key: directpv.min.io/zone
              operator: In
              values:
                - default
  ...
```

If non-standard labels are used to handle the topology the operator exposes an `additionalZoneLabelRegexes` property. For example, if you are using the helm chart to install the operator, you can set the `operator.additionalZoneLabelRegexes` helm value to `directpv.*zone` to match the above PersistentVolume.

### What does the loadBalancerType field on the ClickHouseCluster CRD do?

The `loadBalancerType` doesn't actually add load balancer annotations to the Service of the ClickHouse cluster. Currently, the ClickHouse operator doesn't support creating a load balancer for the ClickHouse cluster. This field only serves a purpose in ClickHouse Cloud where it is used to help create users for the Cloud SQL Console.

### Do I have to use a NVMe attached instance? What purpose does the NVMe disk serve?

The NVMe disk is used by the ClickHouse operator to mount a cache volume in the ClickHouse server pods. This cache volume is used as a cache for data coming from S3 (or equivalent object storages). The operator automatically sets up the disks in the ClickHouse server configuration:

```yaml theme={null}
s3diskWithCache:
    type: cache
    disk: s3disk
    path: /mnt/ClickHouse-cache/sharedS3DiskCache
    max_size:
    '@from_env': CONFIG_DISK_CACHE_SIZE
    cache_on_write_operations: 1
```

Note that if using the SharedMergeTree ClickHouse doesn't actually store data locally. There is no purpose other than optimising the performance of queries executing against the SharedMergeTree.

It is possible to use other storage devices, such as e.g. AWS EBS when using EC2 instances. This changes a few steps in the onboarding guide:

* You need to make sure the disks are attached to the instances used to schedule the ClickHouse server Kubernetes pods
* The launch template should be altered to reflect the changes in disk architecture. It is still possible to create a RAID disk & to format the disk as ext4 or xfs but obviously the devices should be listed with another tool (e.g. lsblk).
* If the disks aren't mounted at `/nvme/disk` like in the example launch template, the `hostPathBaseDirectory` in the ClickHouseCluster helm chart should be set to the actual mount point.

Note that depending on the choices made, you can impact the performance of queries.