Cluster Design Objectives

This section describes some of the work that your cluster will perform and identifies key design considerations.

Begin by understanding the work that the cluster will perform. Establish metrics for data storage capacity, throughput, and characterize the data processing that will typically be performed.

Data Workload

While MapR is relatively easy to install and administer, designing and tuning a large production MapReduce cluster is a complex task that begins with understanding your data needs. Consider the kind of data processing that will occur and estimate the storage capacity and throughput speed required. Data movement, independent of MapReduce operations, is also a consideration. Plan for how data will arrive at the cluster, and how it will be made useful elsewhere.

Network bandwidth and disk I/O speeds are related; either can become a bottleneck. CPU-intensive workloads reduce the relative importance of disk or network speed. If the cluster will be performing a large number of big reduces, network bandwidth is important, suggesting that the hardware plan include multiple NICs per node. The MapR Core can natively take advantage of multiple NICs and distribute workload across them. In general, the more network bandwidth, the faster things will run.

Running NFS on multiple data nodes can improve data transfer performance and make direct loading and unloading of data possible, but multiple NFS instances requires an Converged Enterprise Edition, Hadoop module license. For more information about NFS, see Setting Up MapR NFS.

Plan which nodes will provide NFS access according to your anticipated traffic. For instance, if you need 5Gb/s of write throughput and 5Gb/s of read throughput, the following node configurations would be suitable:

  • 12 NFS nodes with a single 1GbE connection each
  • 6 NFS nodes with dual 1GbE connections each
  • 4 NFS nodes with quadruple 1GbE connections each

When you set up NFS on all of the file server nodes, you enable a self-mounted NFS point for each node. A cluster made up of nodes with self-mounted NFS points enable you to run native applications as tasks. You can use round-robin DNS or a hardware load balancer to mount NFS on one or more dedicated gateways outside the cluster to allow controlled access.

High Availability

A properly licensed and configured MapR cluster provides automatic failover for continuity throughout the MapR Core stack. Configuring a cluster for HA involves redundant instances of specific services, as well as a correct configuration of the MapR NFS service. HA features are not available with the Converged Community Edition.

The following table describes redundant services used for HA:

Service

Strategy

Min. instances

CLDB

Master/slave--two instances in case one fails.

2

ZooKeeper

A majority of ZK nodes (a quorum) must be up.

3

ResourceManager One active and one or more standby instances. If the active one fails, one standby instance takes over. This is configured automatically using Zero Configuration. 2

NFS

The more redundant NFS services, the better.

2

OpenTSDB

At least one instance should be up.

3
Elasticsearch

At least two instances should be up.

3
Note: You should use an odd number of ZooKeeper instances. Setting up more than 5 ZooKeeper instances is not usually needed.

For a high availability cluster, use 5 ZooKeepers, so that the cluster can tolerate 2 ZooKeeper nodes failing and still maintain a quorum.

On a large cluster, you may choose to have extra nodes available in preparation for failover events. In this case, you keep spare, unused nodes ready to replace nodes running control services, such as CLDB or ZooKeeper in case of a hardware failure.

Virtual IP Addresses

You can use virtual IP addresses (VIPs) for load balancing or failover with Converged Enterprise Edition, Hadoop module. VIPs provide multiple addresses that can be leveraged for round-robin DNS, allowing client connections to be distributed among a pool of NFS nodes. VIPs also enable high availability (HA) NFS. In a HA NFS system, when an NFS node fails, data requests are satisfied by other NFS nodes in the pool. Use a minimum of one VIP per NFS node per NIC that clients will use to connect to the NFS server. If you have four nodes with four NICs each, with each NIC connected to an individual IP subnet, use a minimum of 16 VIPs and direct clients to the VIPs in round-robin fashion. The VIPs should be in the same IP subnet as the interfaces to which they will be assigned. See Setting Up VIPs for NFS for details on enabling VIPs for your cluster.

If you plan to use VIPs on your cluster's NFS nodes, consider the following tips:

  • Set up NFS on at least three nodes if possible.
  • All NFS nodes must be accessible over the network from the machines where you want to mount them.
  • To serve a large number of clients, set up dedicated NFS nodes and load-balance between them. If the cluster is behind a firewall, you can provide access through the firewall via a load balancer instead of direct access to each NFS node. You can run NFS on all nodes in the cluster, if needed.
  • To provide maximum bandwidth to a specific client, install the NFS service directly on the client machine. The NFS gateway on the client manages how data is sent in or read back from the cluster, using all its network interfaces (that are on the same subnet as the cluster nodes) to transfer data via MapR APIs, balancing operations among nodes as needed.
  • Use VIPs to provide High Availability (HA) and failover.