Home > Storage > ObjectScale and ECS > Product Documentation > Dell ObjectScale: Overview and Architecture > Object store: logical topology
In a Kubernetes cluster, a node is a physical worker node. In ObjectScale, a node is referred to as a storage server (SS) instance. An SS provides disk access in an object store. At most, one SS instance from each object store is scheduled on a Kubernetes node.
SS instances are used to store user object data, which includes any associated user object metadata, and system object metadata, such as where an object is stored on disk.
The following figure shows five Kubernetes nodes, N1-N5, in the Kubernetes cluster. The figure shows two object stores in the 5-node Kubernetes cluster. Of the five nodes, N3 and N4 are used by both object stores. Three Kubernetes nodes are each used by only one of the object stores. Objectstore_1 uses N1-N4. Objectstore_2 uses N3-N5. The object stores share the physical node resources of N3 and N4 but access storage media independently within their own set of reserved resources. Resource reservations for Objectstore_1 and Objectstore_2 allow the object stores to run side by side on N3 and N4 in the Kubernetes cluster.
The following figure depicts the Kubernetes and object store topology trees from the preceding example. Each object store is only aware of its own topology.
The following figure shows the relationship between disks and SS instances for this example. Objectstore_1 and Objectstore_2 SS instances independently access dedicated volumes and disks for data storage on Kubernetes nodes.
The figure shows each SS instance connected to a single disk for simplicity. The key point to understand is that each SS instance in an object store is tied to a single Kubernetes node. SS instances are assigned to be scheduled on a Kubernetes node. During node failure, the SS instance and underlying persistent storage are lost. The data segments in the persistent volumes that are lost are re-created across other nodes. This functionality contrasts with a common Kubernetes behavior where instances that are lost during node failure are brought up elsewhere. The ephemeral SS instances are associated with the persistent storage that they attach to in ObjectScale.