Optimizing Storage | Scaling and Performance Guide | OpenShift Container Platform 3.6

Overview
General Storage Guidelines
Storage Recommendations
- Specific Application Storage Recommendations
- Other Specific Application Storage Recommendations
Choosing a Graph Driver
- Benefits of Using the OverlayFS Versus DeviceMapper with SELinux
- Comparing the Overlay Versus overlay2 Graph Drivers

Overview

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.

General Storage Guidelines

The following table lists the available persistent storage technologies for OpenShift Container Platform.

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	CNS/CRS GlusterFS ^[1] iSCSI, Fibre Channel, Ceph RBD, OpenStack Cinder, AWS EBS ^[1], Dell/EMC Scale.IO, VMware vSphere Volume, GCE Persistent Disk ^[1], Azure Disk
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.	CNS/CRS GlusterFS ^[1], RHEL NFS, NetApp NFS ^[2] , Azure File, Vendor NFS, Vendor GlusterFS ^[3], Azure File, AWS EFS
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.	CNS/CRS GlusterFS ^[1], Ceph Object Storage (RADOS Gateway), OpenStack Swift, Aliyun OSS, AWS S3, Google Cloud Storage, Azure Blob Storage, Vendor S3 ^[3], Vendor Swift ^[3]

As of OpenShift Container Platform 3.6.1, Container-Native Storage (CNS) GlusterFS (a hyperconverged or cluster-hosted storage solution) and Container-Ready Storage (CRS) GlusterFS (an externally hosted storage solution) provides interfaces for block, file, and object storage for the purpose of the OpenShift Container Platform registry, logging, and metrics.

Storage Recommendations

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	ROX ^[4]	RWX ^[5]	Registry	Scaled registry	Metrics	Logging	Apps
Block	Yes ^[6]	No	Configurable	Not configurable	Recommended	Recommended	Recommended
File	Yes ^[6]	Yes	Configurable	Configurable	Configurable	Configurable	Recommended
Object	Yes	Yes	Recommended	Recommended	Not configurable	Not configurable	Not configurable ^[7]

A scaled registry is an OpenShift Container Platform registry where three or more pod replicas are running.

Specific Application Storage Recommendations

Registry

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

The preferred storage technology is object storage followed by block storage. The storage technology does not need to support RWX access mode.
The storage technology must ensure read-after-write consistency. All NAS storage (excluding CNS/CRS GlusterFS as it uses an object storage interface) are not recommended for OpenShift Container Platform Registry cluster deployment with production workloads.
While hostPath volumes are configurable for a non-scaled/HA OpenShift Container Platform Registry, they are not recommended for cluster deployment.

Corruption may occur when using NFS to back OpenShift Container Platform registry with production workloads.

Scaled Registry

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

The preferred storage technology is object storage. The storage technology must support RWX access mode and must ensure read-after-write consistency.
File storage and block storage are not recommended for a scaled/HA OpenShift Container Platform registry cluster deployment with production workloads.
All NAS storage (excluding CNS/CRS GlusterFS, as it uses an object storage interface) are not recommended for OpenShift Container Platform Registry cluster deployment with production workloads.

Corruption may occur when using NFS to back an OpenShift Container Platform scaled/HA registry with production workloads.

Metrics

In an OpenShift Container Platform hosted metrics cluster deployment:

The preferred storage technology is block storage.
It is not recommended to use NAS storage (excluding CNS/CRS GlusterFS as it uses a block storage interface from iSCSI) for a hosted metrics cluster deployment with production workloads.

Corruption may occur when using NFS to back 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.
It is not recommended to use NAS storage (excluding CNS/CRS GlusterFS as it uses a block storage interface from iSCSI) for a hosted metrics cluster deployment with production workloads.

Corruption may occur when using NFS to back hosted logging with production workloads.

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.
NFS does not guarantee read-after-write consistency and is not recommended for applications which require it.
Applications that depend on writing to the same, shared NFS export may experience issues with production workloads.

Other Specific Application Storage Recommendations

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

Choosing a Graph Driver

Docker stores images and containers in a graph driver (a pluggable storage technology), such as DeviceMapper, Overlay, and Btrfs. Each has advantages and disadvantages. For example, Overlay is faster than DeviceMapper at starting and stopping containers, but is not Portable Operating System Interface for Unix (POSIX) compliant because of the architectural limitations of a union file system, and does not yet support SELinux.

For more information about OverlayFS, including supportability and usage caveats, see the Red Hat Enterprise Linux (RHEL) 7 Release Notes for your version.

In production environments, using a LVM thin pool on top of regular block devices (not loop devices) for container images and container root file systems storage is recommended.

Using a loop device can affect performance issues. While you can still continue to use it, Docker logs the following warning message:

devmapper: Usage of loopback devices is strongly discouraged for production use.
Please use `--storage-opt dm.thinpooldev` or use `man docker` to refer to
dm.thinpooldev section.

To ease Docker backend storage configuration, use the docker-storage-setup utility, which automates much of the configuration details:

If you had a separate disk drive dedicated to Docker storage (for example, /dev/xvdb), add the following to the /etc/sysconfig/docker-storage-setup file:
```
DEVS=/dev/xvdb
VG=docker_vg
```
Restart the docker-storage-setup service:
```
# systemctl restart docker-storage-setup
```
After the restart, docker-storage-setup sets up a volume group named docker_vg and creates a thin-pool logical volume. Documentation for thin provisioning on RHEL is available in the LVM Administrator Guide. View the newly created volumes with the lsblk command:
```
# lsblk /dev/xvdb
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvdb 202:16 0 20G 0 disk
└─xvdb1 202:17 0 10G 0 part
  ├─docker_vg-docker--pool_tmeta 253:0 0 12M 0 lvm
  │ └─docker_vg-docker--pool 253:2 0 6.9G 0 lvm
  └─docker_vg-docker--pool_tdata 253:1 0 6.9G 0 lvm
  └─docker_vg-docker--pool 253:2 0 6.9G 0 lvm
```
Thin-provisioned volumes are not mounted and have no file system (individual containers do have an XFS file system), thus they do not show up in df output.

To verify that Docker is using an LVM thin pool, and to monitor disk space utilization, use the docker info command. The Pool Name corresponds with the VG you specified in /etc/sysconfig/docker-storage-setup:

# docker info | egrep -i 'storage|pool|space|filesystem'
Storage Driver: devicemapper
 Pool Name: docker_vg-docker--pool
 Pool Blocksize: 524.3 kB
 Backing Filesystem: xfs
 Data Space Used: 62.39 MB
 Data Space Total: 6.434 GB
 Data Space Available: 6.372 GB
 Metadata Space Used: 40.96 kB
 Metadata Space Total: 16.78 MB
 Metadata Space Available: 16.74 MB

By default, a thin pool is configured to use 40% of the underlying block device. As you use the storage, LVM automatically extends the thin pool up to 100%. This is why the Data Space Total value does not match the full size of the underlying LVM device. This auto-extend technique was used to unify the storage approach taken in both Red Hat Enterprise Linux and Red Hat Atomic Host, which only uses a single partition.

In development, Docker in Red Hat distributions defaults to a loopback mounted sparse file. To see if your system is using the loopback mode:

# docker info|grep loop0
 Data file: /dev/loop0

Red Hat strongly recommends using the DeviceMapper storage driver in thin-pool mode for production workloads.

OverlayFS is also supported for container runtimes use cases as of Red Hat Enterprise Linux 7.2, and provides faster start up time and page cache sharing, which can potentially improve density by reducing overall memory utilization.

Benefits of Using the OverlayFS Versus DeviceMapper with SELinux

The main advantage of the OverlayFS graph is Linux page cache sharing among containers that share an image on the same node. This attribute of OverlayFS leads to reduced input/output (I/O) during container startup (and, thus, faster container startup time by several hundred milliseconds), as well as reduced memory usage when similar images are running on a node. Both of these results are beneficial in many environments, especially those with the goal of optimizing for density and have high container churn rate (such as a build farm), or those that have significant overlap in image content.

Page cache sharing is not possible with DeviceMapper because thin-provisioned devices are allocated on a per-container basis.

DeviceMapper is the default Docker storage configuration on Red Hat Enterprise Linux. The use of OverlayFS as the container storage technology is under evaluation and moving Red Hat Enterprise Linux to OverlayFS as the default in future releases is under consideration.

Comparing the Overlay Versus overlay2 Graph Drivers

OverlayFS is a type of union file system. It allows you to overlay one file system on top of another. Changes are recorded in the upper file system, while the lower file system remains unmodified. This allows multiple users to share a file-system image, such as a container or a DVD-ROM, where the base image is on read-only media.

OverlayFS layers two directories on a single Linux host and presents them as a single directory. These directories are called layers, and the unification process is referred to as a union mount.

OverlayFS uses one of two graph drivers, overlay or overlay2. As of Red Hat Enterprise Linux 7.2, overlay became a supported graph driver. As of Red Hat Enterprise Linux 7.4, overlay2 became supported. SELinux on the docker daemon became supported in Red Hat Enterprise Linux 7.4. See the Red Hat Enterprise Linux release notes for information on using OverlayFS with your version of RHEL, including supportability and usage caveats.

The overlay2 driver natively supports up to 128 lower OverlayFS layers but, the overlay driver works only with a single lower OverlayFS layer. Because of this capability, the overlay2 driver provides better performance for layer-related Docker commands, such as docker build, and consumes fewer inodes on the backing filesystem.

Because the overlay driver works with a single lower OverlayFS layer, you cannot implement multi-layered images as multiple OverlayFS layers. Instead, each image layer is implemented as its own directory under /var/lib/docker/overlay. Hard links are then used as a space-efficient way to reference data shared with lower layers.

Docker recommends using the overlay2 driver with OverlayFS rather than the overlay driver, because it is more efficient in terms of inode utilization.

To use overlay2 with RHEL or CentOS you need version 3.10.0-693 or higher of the kernel.

1. CNS/CRS GlusterFS, Ceph RBD, OpenStack Cinder, AWS EBS, Azure Disk, GCE persistent disk, and VMware vSphere support dynamic persistent volume (PV) provisioning natively in OpenShift Container Platform.

2. NetApp NFS supports dynamic PV provisioning when using the Trident plugin.

3. Vendor GlusterFS, Vendor S3, and Vendor Swift supportability and configurability may vary.

4. ReadOnlyMany

5. ReadWriteMany

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

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.