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Introduction to Ceph


Ceph is an open-source, distributed storage system that provides object, block and file storage interfaces from a single cluster. In this guide we give a high-level overview of Ceph's services and how they are used in your OpenMetal Private Cloud.

Advantages of Ceph

See Ceph's Benefits page for a more complete understanding of the benefits Ceph provides.

Data Resiliency

With Ceph, your data storage is resilient either through the use of replication or erasure coding. In our configuration, replication is used to create multiple copies of data. Data is replicated at the host level. A Private Cloud is deployed with three hosts. With our Ceph configuration, you could lose two hosts and still have all of your Ceph cluster's data.

Ceph Scales Extremely Well

Ceph is designed to scale horizontally into Petabyte figures.

Disadvantages of Ceph

Data access times are not as quick compared to accessing data directly from a disk. Should your workload require very fast disk reads and writes, consider using a compute only node, where instances can be spun up on local, LVM-backed storage.

For more, see Spin up an Instance with Ephemeral Storage.

Ceph Version Used by Private Clouds

With current Private Cloud deployments, Ceph's Octopus release is used.

View Disk Usage of the Ceph Cluster

For more, see the guide on How to Check Ceph's Status and Disk Usage.

Default Configuration for the Ceph Cluster

Core Ceph services are deployed to each control plane node upon initial cloud deployment. This means each node comes installed with Ceph's Monitor, Manager, Object Gateway (RADOSGW), and Object Storage Daemon (OSD) services. Each node's secondary NVMe drive backs a Ceph OSD. Our Ceph deployments are configured with host-level replication of data. Your cloud can lose all but one host and still retain all of the Ceph cluster's data.

Default Ceph Pools

By default, your Ceph cluster's data storage is logically divided into pools, a concept associated with Ceph.

From a control plane node as root, we use ceph osd lspools to list the default pools associated with a Private Cloud:

1 device_health_metrics
2 images
3 volumes
4 vms
5 backups
6 metrics
7 manila_data
8 manila_metadata
9 .rgw.root
10 default.rgw.log
11 default.rgw.control
12 default.rgw.meta
13 default.rgw.buckets.index

Next, we explain the purpose of some of the Ceph pools.

Pool: images

Operating System images maintained by Glance are stored in this pool

Pool: volumes

Cinder stores any volumes created by your cloud in this pool.

Pool: vms

When you spin up a volume-backed instance, Nova is configured to create a volume for that instance in this pool.

Pool: backups

Cinder stores volume backups within this pool.

Swift and Cinder Ceph Configuration

With Private Clouds, Swift and Cinder are the services configured to connect to Ceph.

Swift, OpenStack's Object Storage service, connects directly to the Ceph cluster. With our setup you will not see configuration for Swift in a place like /etc/kolla/swift. This configuration is instead handled by Ceph directly and can be viewed through /etc/ceph/ceph.conf.

Cinder, OpenStack's Block Storage service, is also configured to connect to Ceph. With Cinder, the are several services, of which cinder-volume and cinder-backup are connected to Ceph. The Ceph configuration for each service can be viewed through /etc/kolla/cinder-volume/cinder.conf and /etc/kolla/cinder-backup/cinder.conf.

Reconfiguring your Ceph Cluster

WARNING! -- Our current deployment system deploys a Private Cloud with a known working state. Should you deviate from this state by adjusting your cloud's Ceph configuration you can no longer safely use the functions in OpenMetal Central to add nodes to your cloud or add IP blocks. Should you use these functions, any custom configurations to Ceph will be reverted. We are working on rolling out a new deployment system allowing custom cloud configurations. We can still add new nodes and IP blocks to your cloud but must do so manually. Please reach out to your Account Manager should this apply to you.

Your Ceph cluster was deployed using Ceph Ansible. Any configuration changes must be made using Ceph Ansible. For more information, see How to Prepare and Use Ceph Ansible