2015-08-31 10:20 PM
Quick examples... for a deeper dive, I recommend taking the free classes on learningcenter.netapp.com and also check the ILT and WBT courses offered by NetApp and NetApp training partners.
Per SVM (Storage Virtual Machine) there is a namespare or new root mount point you build or stitch together with volumes mounted to junctions. Related to junction examples below.
A junction is where a volume is mounted to the namespace. vol1 to /vol1 vol2 to /vol1/vol2 or to /vol2 for example. The path the user sees the volume in the namespace.
Example: A 4-node cluster is in quorum with any 3 nodes. Or 2 nodes as long as one of the 2 nodes has epsilon
Assigned to one node only in a cluster 4 or more nodes (2 node clusters disable epsilon...growing to 4 nodes or more enables epsilon). Basically it is the heavier weight node...from above if a 4 node cluster is split 2 and 2... the 2 nodes with epsilon stay in quorum. The other 2 are out of quorum.
2015-08-31 10:33 PM
Understanding quorum and epsilon:
Quorum and epsilon are important measures of cluster health and function that together indicate how clusters address potential communications and connectivity challenges.
Quorum is a precondition for a fully-functioning cluster. When a cluster is in quorum, a simple majority of nodes are healthy and can communicate with each other. When quorum is lost, the cluster loses the ability to accomplish normal cluster operations. Only one collection of nodes can have quorum at any one time because all of the nodes collectively share a single view of the data. Therefore, if two non-communicating nodes are permitted to modify the data in divergent ways, it is no longer possible to reconcile the data into a single data view.
Each node in the cluster participates in a voting protocol that elects one node master; each remaining node is a secondary. The master node is responsible for synchronizing information across the cluster. When quorum is formed, it is maintained by continual voting; if the master node goes offline, a new master is elected by the nodes that remain online.
Because there is the possibility of a tie in a cluster that has an even number of nodes, one node has an extra fractional voting weight called epsilon. When the connectivity between two equal portions of a large cluster fails, the group of nodes containing epsilon maintains quorum, assuming that all of the nodes are healthy. For example, if a single link is established between 12 nodes in one room and 12 nodes in another room to compose a 24-node cluster and the link fails, then the group of nodes that holds epsilon would maintain quorum and continue to serve data while the other 12 nodes would stop serving data. However, if the node holding epsilon was unhealthy or offline, then quorum would not be formed, and all of the nodes would stop serving data.
Epsilon is automatically assigned to the first node when the cluster is created. If the node that holds epsilon becomes unhealthy or is taken over by its high availability partner, epsilon does not move to another node but is rather no longer a factor in determining quorum.
In general, assuming reliable connectivity among the nodes of the cluster, a larger cluster is more stable than a smaller cluster. The quorum requirement of a simple majority of half the nodes plus epsilon is easier to maintain in a cluster of 24 nodes than in a cluster of two nodes.
A two-node cluster presents some unique challenges for maintaining quorum. In a two-node cluster, neither node holds epsilon; instead, both nodes are continuously polled to ensure that if one node fails, the other has full read-write access to data, as well as access to logical interfaces and management functions.
Junction path :
Volume junctions function to join individual volumes together, into a single logical namespace, in order to enable data access to NAS clients.
When NAS clients access data by traversing a junction, the junction appears to be an ordinary directory. A junction is formed when a volume is mounted to a mount point below the root, and is used to create a file-system tree. The top of a file-system tree is always the root volume, which is represented by a slash (/). A junction leads from a directory in one volume to the root directory of another volume.
For more details refer KB article, https://kb.netapp.com/support/index?page=content&id=3014349&actp=search&viewlocale=en_US&searchid=1441084594899
A namespace consists of a group of volumes connected using junction paths. It is the hierarchy of flexible volumes within a single SVM as presented to NAS clients.
As storage or performance requirements for an SVM change, the SVM’s namespace can adapt accordingly. Additional flexible volumes can be added nondisruptively to increase capacity, with no client-side configuration changes. Flexible volumes in an SVM with higher performance requirements can be physically moved nondisruptively to aggregates with faster storage. Flexible volumes can be physically moved to different aggregates inside the cluster without affecting their logical location in the SVM’s namespace.
2015-08-31 10:49 PM
Name space: A namespace is a file system. Every FlexVol within an SVM presents a single directory hierarchical view and has a unique namespace. The namespace enables NAS clients to access data without specifying the physical location of the data. The namespace also enable the cluster and SVM admins to manage distributed data storages as a single directory with multiple levels of hierarchy.
Junction: volume junctions are a way to join individual volume together into a single logical namespace. Volume junctions are transparent to CIFS and NFS clients. When NAS clients access data by traversing a junction, the junction appears to be an ordinary directory.
Quorum: A Quorum is a simple majority of connected, healthy and eligible nodes. The members of each RDB unit vote to determine which node will be their master.
Epsilon: One node in the cluster has a special voting weight called Epsilon. Unlike the masters of each RDB unit which might be different, the epsilon node is the same for all RDB units. This epsilon vote is used only in the case of an even partitioning of a cluster, where, for example, four nodes of an eight node cluster cannot talk to the other four nodes. PS: This Situation is rare.