The Dell EMC Ready Architecture for Red Hat OpenStack Platform includes Red Hat Ceph storage, which is a scale-out, distributed, software-defined storage system. Red Hat Ceph storage is used as backend storage for Nova, Cinder and Glance. Storage nodes run the Red Hat Ceph storage software, and Compute and Controller nodes run the Red Hat Ceph storage block client.
Red Hat Ceph storage also provides object storage for OpenStack VMs and other clients external to OpenStack. The object storage interface is an implementation of:
- The OpenStack Swift RESTful API (basic data access model)
- The Amazon S3 RESTful API
The object storage interface is provided by Red Hat Ceph storage RADOS Gateway software running on the Controller nodes. Client access to the object storage is distributed across all Controller nodes in order to provide HA and IO load balancing.
Red Hat Ceph storage is containerized in RHOSP16.1 (Mon, Mgr, Object Gateway, and Object Storage Daemon). Each OSD has an associated physical drive where the data is stored, and a journal where write operations are staged prior to being committed.
- When a client reads data from the Red Hat Ceph storage cluster the OSDs fetch the data directly from the drives.
- When a client writes data to the storage cluster the OSDs write the data to their journals prior to committing the data.
OSD journals can be located in a separate partition on the same physical drive where the data is stored, or they can be located on a separate high-performance drive, such as an SSD optimized for write-intensive workloads. For the Architecture Guide, a recommended ratio of one (1) SSD to four (4) hard disks is used to achieve optimal performance. It should be noted that as of this writing, using a greater ratio will result in server performance degradation.
In a cost-optimized solution, the greatest storage density is achieved by forgoing separate SSD journal drives, and populating every available physical drive bay with a high-capacity HDD.
In a throughput-optimized solution, a few drive bays can be populated with high performance SSDs that will host the journals for the OSD HDDs. For example, in an Dell EMC PowerEdge R740xd system with 20 drive bays available for Red Hat Ceph storage, 4 bays are used for SSD journal drives and 16 bays for HDD data drives. This is based upon the current industry guideline of a 4:1 HDD to SSD journal ratio.