Optimizing persistent storage

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 OKD.

You can use Containerized GlusterFS GlusterFS (a hyperconverged or cluster-hosted storage solution) or External GlusterFS GlusterFS (an externally hosted storage solution) for block, file, and object storage for OKD registry, logging, and monitoring.

Storage recommendations

The following table summarizes the recommended and configurable storage technologies for the given OKD cluster application.

Specific application storage recommendations

Registry

In a non-scaled/high-availability (HA) OKD 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 Containerized GlusterFS/External GlusterFS GlusterFS as it uses an object storage interface) are not recommended for OKD Registry cluster deployment with production workloads.

• While hostPath volumes are configurable for a non-scaled/HA OKD Registry, they are not recommended for cluster deployment.

Scaled registry

In a scaled/HA OKD 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 OKD registry cluster deployment with production workloads.

• All NAS storage (excluding Containerized GlusterFS/External GlusterFS GlusterFS as it uses an object storage interface) are not recommended for OKD Registry cluster deployment with production workloads.

Monitoring

In an OKD hosted monitoring cluster deployment:

• The preferred storage technology is block storage.

• If you decide to configure file storage, make sure that it follows POSIX standards.

Logging

In an OKD hosted logging cluster deployment:

• The preferred storage technology is block storage.

• It is not recommended to use NAS storage (excluding Containerized GlusterFS/External GlusterFS GlusterFS as it uses a block storage interface from iSCSI) for a hosted metrics cluster deployment 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.

• 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

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

• 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 OverlayFS. Each has advantages and disadvantages.

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

For better performance, Red Hat strongly recommends using the overlayFS storage driver over Device Mapper. However, if you are already using Device Mapper in a production environment, Red Hat strongly recommends using thin provisioning for container images and container root file systems. Otherwise, always use overlayfs2 for Docker engine or overlayFS for CRI-O.

Using a loop device can affect performance. 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:

For Overlay

1. Edit the /etc/sysconfig/docker-storage-setup file to specify the device driver:

   STORAGE_DRIVER=overlay2

   If using CRI-O, specify STORAGE_DRIVER=overlay. With CRI-O, the default overlay storage driver uses the overlay2 optimizations.

   With OverlayFS, if you want to have imagefs on a different logical volume, then you must set CONTAINER_ROOT_LV_NAME and CONTAINER _ROOT_LV_MOUNT_PATH. Setting CONTAINER_ROOT_LV_MOUNT_PATH requires CONTAINER_ROOT_LV_NAME to be set. For example, CONTAINER_ROOT_LV_NAME="container-root-lv". See Using the Overlay Graph Driver for more information.

2. 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

3. Restart the docker-storage-setup service:

   # systemctl restart docker-storage-setup

4. To verify that docker is using overlay2, and to monitor disk space use, run the docker info command:

   # docker info | egrep -i 'storage|pool|space|filesystem'
   Storage Driver: overlay2 (1)
    Backing Filesystem: extfs

   1	The docker info output when using overlay2.

   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.

For Thinpool

1. Edit the /etc/sysconfig/docker-storage-setup file to specify the device driver:

   STORAGE_DRIVER=devicemapper

2. 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

3. 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.

4. To verify that docker is using an LVM thinpool, and to monitor disk space use, run the docker info command:

   # docker info | egrep -i 'storage|pool|space|filesystem'
   Storage Driver: devicemapper (1)
    Pool Name: docker_vg-docker--pool (2)
    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

   1	The docker info output when using devicemapper.
   2	Corresponds to the VG you specified in /etc/sysconfig/docker-storage-setup.

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.

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

Benefits of using OverlayFS or 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.

Comparing the Overlay and 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.

1. This does not apply to physical disk, VM physical disk, VMDK, loopback over NFS, AWS EBS, Azure Disk and Cinder (the latter for block).