Optimizing Storage | Scaling and Performance Guide

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 Origin.

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 Origin 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 Origin 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 Origin registry, logging, and metrics.

Storage Recommendations

The following table summarizes the recommended and configurable storage technologies for the given OpenShift Origin 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 Origin registry where three or more pod replicas are running.

Specific Application Storage Recommendations

Registry

In a non-scaled/high-availability (HA) OpenShift Origin 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 Origin Registry cluster deployment with production workloads.
While hostPath volumes are configurable for a non-scaled/HA OpenShift Origin Registry, they are not recommended for cluster deployment.

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

Scaled Registry

In a scaled/HA OpenShift Origin 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 Origin 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 Origin Registry cluster deployment with production workloads.

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

Metrics

In an OpenShift Origin 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 Origin 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 Origin 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

Container runtimes store images and containers in a graph driver (a pluggable storage technology), such as DeviceMapper and`Overlay`. Each has advantages and disadvantages.

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

Table 3. Graph Driver Comparisons
Name	Description	Benefits	Limitations
Device Mapper loop-lvm	Uses the Device Mapper thin provisioning module (dm-thin-pool) to implement copy-on-write (CoW) snapshots. For each device mapper graph location, thin pool is created based on two block devices, one for data and one for metadata. By default, these block devices are created automatically by using loopback mounts of automatically created sparse files.	It works out of the box, so it is useful for prototyping and development purposes.	Not all Portable Operating System Interface for Unix (POSIX) features work (for example, `O_DIRECT`). Most importantly, this mode is unsupported for production workloads. All containers and images share the same pool of capacity. It cannot be resized without destroying and re-creating the pool.
Device Mapper Thin Provisioning	Also uses LVM, Device Mapper, and the dm-thinp kernel module. It differs by removing the loopback device, talking straight to a raw partition (no filesystem).	There are measurable performance advantages at moderate load and high density. It gives you per-container limits for capacity (10G by default).	You have to have a dedicated partition for it. It is not set up by default in Red Hat Enterprise Linux (RHEL). All containers and images share the same pool of capacity. It cannot be resized without destroying and re-creating the pool.
OverlayFS	Combines a lower (parent) and upper (child) filesystem and a working directory (on the same filesystem as the child). The lower filesystem is the base image, and when you create new containers, a new upper filesystem is created containing the deltas.	Faster than Device Mapper at starting and stopping containers. The startup time difference between Device Mapper and Overlay is generally less than one second. Allows for page cache sharing.	Not POSIX compliant.

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

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

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

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 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.

Overlay 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 Overlay Graph Driver with SELinux

The default Docker storage configuration on Red Hat Enterprise Linux continues to be DeviceMapper. While the use of Overlay as the container’s storage technology is under evaluation, moving Red Hat Enterprise Linux to Overlay as the default in future releases is under consideration. As of Red Hat Enterprise Linux 7.2, Overlay became a supported graph driver. As of Red Hat Enterprise Linux 7.4, SELinux and the Overlay2 graph driver became a supported combination.

The main advantage of the Overlay file system is Linux page cache sharing among containers sharing an image on the same node. This attribute of Overlay 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.

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 Origin.

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 Origin’s PVs/persistent volume claims (PVCs). Apps must integrate with the object storage REST API.