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Optimizing storage

Optimizing storage helps to minimize storage use across all resources. By optimizing storage, administrators help ensure that existing storage resources are working in an efficient manner.

Available persistent storage options

Understand your persistent storage options so that you can optimize your OpenShift Container Platform environment.

Table 1. Available storage options
Storage type Description Examples

Block

  • Presented to the operating system (OS) as a block device

  • Suitable for applications that need full control of storage and operate at a low level on files bypassing the file system

  • Also referred to as a Storage Area Network (SAN)

  • Non-shareable, which means that only one client at a time can mount an endpoint of this type

AWS EBS and VMware vSphere support dynamic persistent volume (PV) provisioning natively in OpenShift Container Platform.

File

  • Presented to the OS as a file system export to be mounted

  • Also referred to as Network Attached Storage (NAS)

  • Concurrency, latency, file locking mechanisms, and other capabilities vary widely between protocols, implementations, vendors, and scales.

RHEL NFS, NetApp NFS [1], and Vendor NFS

Object

  • Accessible through a REST API endpoint

  • Configurable for use in the OpenShift Container Platform Registry

  • Applications must build their drivers into the application and/or container.

AWS S3

Currently, CNS is not supported in OpenShift Container Platform 4.3.

The following table summarizes the recommended and configurable storage technologies for the given OpenShift Container Platform cluster application.

Table 2. Recommended and configurable storage technology
Storage type ROX1 RWX2 Registry Scaled registry Metrics3 Logging Apps

Block

Yes4

No

Configurable

Not configurable

Recommended

Recommended

Recommended

File

Yes4

Yes

Configurable

Configurable

Configurable5

Configurable6

Recommended

Object

Yes

Yes

Recommended

Recommended

Not configurable

Not configurable

Not configurable7

1 ReadOnlyMany

2 ReadWriteMany.

3 Prometheus is the underlying technology used for metrics.

4 This does not apply to physical disk, VM physical disk, VMDK, loopback over NFS, AWS EBS, and Azure Disk.

5 For metrics, using file storage with the ReadWriteMany (RWX) access mode is unreliable. If you use file storage, do not configure the RWX access mode on any PersistentVolumeClaims that are configured for use with metrics.

6 For logging, using any shared storage would be an anti-pattern. One volume per elasticsearch is required.

7 Object storage is not consumed through OpenShift Container Platform’s PVs/persistent volume claims (PVCs). Apps must integrate with the object storage REST API.

A scaled registry is an OpenShift Container Platform registry where two or more Pod replicas are running.

Specific application storage recommendations

Testing shows issues with using the NFS server on RHEL as storage backend for core services. This includes the OpenShift Container Registry and Quay, Prometheus for monitoring storage, and Elasticsearch for logging storage. Therefore, using RHEL NFS to back PVs used by core services is not recommended.

Other NFS implementations on the marketplace might not have these issues. Contact the individual NFS implementation vendor for more information on any testing that was possibly completed against these OpenShift Container Platform core components.

Registry

In a non-scaled/high-availability (HA) OpenShift Container Platform registry cluster deployment:

  • The storage technology does not have to support RWX access mode.

  • The storage technology must ensure read-after-write consistency.

  • The preferred storage technology is object storage followed by block storage.

  • File storage is not recommended for OpenShift Container Platform registry cluster deployment with production workloads.

Scaled registry

In a scaled/HA OpenShift Container Platform registry cluster deployment:

  • The storage technology must support RWX access mode and must ensure read-after-write consistency.

  • The preferred storage technology is object storage.

  • Amazon Simple Storage Service (Amazon S3), Google Cloud Storage (GCS), Microsoft Azure Blob Storage, and OpenStack Swift are supported.

  • Storage should be S3 or Swift compliant.

  • File storage is not recommended for a scaled/HA OpenShift Container Platform registry cluster deployment with production workloads.

  • For non-cloud platforms, such as vSphere and bare metal installations, the only configurable technology is file storage.

  • Block storage is not configurable.

Metrics

In an OpenShift Container Platform hosted metrics cluster deployment:

  • The preferred storage technology is block storage.

  • Object storage is not configurable.

It is not recommended to use file storage for a hosted metrics cluster deployment with production workloads.

Logging

In an OpenShift Container Platform hosted logging cluster deployment:

  • The preferred storage technology is block storage.

  • File storage is not recommended for a scaled/HA OpenShift Container Platform registry cluster deployment with production workloads.

  • Object storage is not configurable.

Testing shows issues with using the NFS server on RHEL as storage backend for core services. This includes Elasticsearch for logging storage. Therefore, using RHEL NFS to back PVs used by core services is not recommended.

Other NFS implementations on the marketplace might not have these issues. Contact the individual NFS implementation vendor for more information on any testing that was possibly completed against these OpenShift Container Platform core components.

Applications

Application use cases vary from application to application, as described in the following examples:

  • Storage technologies that support dynamic PV provisioning have low mount time latencies, and are not tied to nodes to support a healthy cluster.

  • Application developers are responsible for knowing and understanding the storage requirements for their application, and how it works with the provided storage to ensure that issues do not occur when an application scales or interacts with the storage layer.

Other specific application storage recommendations

  • OpenShift Container Platform Internal etcd: For the best etcd reliability, the lowest consistent latency storage technology is preferable.

  • It is highly recommended that you use etcd with storage that handles serial writes (fsync) quickly, such as NVMe or SSD. Ceph, NFS, and spinning disks are not recommended.

  • Red Hat OpenStack Platform (RHOSP) Cinder: RHOSP Cinder tends to be adept in ROX access mode use cases.

  • Databases: Databases (RDBMSs, NoSQL DBs, etc.) tend to perform best with dedicated block storage.

Data storage management

The following table summarizes the main directories that OpenShift Container Platform components write data to.

Table 3. Main directories for storing OpenShift Container Platform data
Directory Notes Sizing Expected growth

/var/lib/etcd

Used for etcd storage when storing the database.

Less than 20 GB.

Database can grow up to 8 GB.

Will grow slowly with the environment. Only storing metadata.

Additional 20-25 GB for every additional 8 GB of memory.

/var/lib/containers

This is the mount point for the CRI-O runtime. Storage used for active container runtimes, including pods, and storage of local images. Not used for registry storage.

50 GB for a node with 16 GB memory. Note that this sizing should not be used to determine minimum cluster requirements.

Additional 20-25 GB for every additional 8 GB of memory.

Growth is limited by capacity for running containers.

/var/lib/kubelet

Ephemeral volume storage for pods. This includes anything external that is mounted into a container at runtime. Includes environment variables, kube secrets, and data volumes not backed by persistent volumes.

Varies

Minimal if pods requiring storage are using persistent volumes. If using ephemeral storage, this can grow quickly.

/var/log

Log files for all components.

10 to 30 GB.

Log files can grow quickly; size can be managed by growing disks or by using log rotate.
1. NetApp NFS supports dynamic PV provisioning when using the Trident plug-in.