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1.2.3. Distributed Systems

To fully realize horizontal scalability, Red Hat Enterprise Linux uses many components of distributed computing. The technologies that make up distributed computing are divided into three layers:
Communication
Horizontal scalability requires many tasks to be performed simultaneously (in parallel). As such, these tasks must have interprocess communication to coordinate their work. Further, a platform with horizontal scalability should be able to share tasks across multiple systems.
Storage
Storage via local disks is not sufficient in addressing the requirements of horizontal scalability. Some form of distributed or shared storage is needed, one with a layer of abstraction that allows a single storage volume's capacity to grow seamlessly with the addition of new storage hardware.
Management
The most important duty in distributed computing is the management layer. This management layer coordinates all software and hardware components, efficiently managing communication, storage, and the usage of shared resources.
The following sections describe the technologies within each layer in more detail.

1.2.3.1. Communication

The communication layer ensures the transport of data, and is composed of two parts:
  • Hardware
  • Software
The simplest (and fastest) way for multiple systems to communicate is through shared memory. This entails the usage of familiar memory read/write operations; shared memory has the high bandwidth, low latency, and low overhead of ordinary memory read/write operations.
Ethernet

The most common way of communicating between computers is over Ethernet. Today, Gigabit Ethernet (GbE) is provided by default on systems, and most servers include 2-4 ports of Gigabit Ethernet. GbE provides good bandwidth and latency. This is the foundation of most distributed systems in use today. Even when systems include faster network hardware, it is still common to use GbE for a dedicated management interface.

10GbE

Ten Gigabit Ethernet (10GbE) is rapidly growing in acceptance for high end and even mid-range servers. 10GbE provides ten times the bandwidth of GbE. One of its major advantages is with modern multi-core processors, where it restores the balance between communication and computing. You can compare a single core system using GbE to an eight core system using 10GbE. Used in this way, 10GbE is especially valuable for maintaining overall system performance and avoiding communication bottlenecks.

Unfortunately, 10GbE is expensive. While the cost of 10GbE NICs has come down, the price of interconnect (especially fibre optics) remains high, and 10GbE network switches are extremely expensive. We can expect these prices to decline over time, but 10GbE today is most heavily used in server room backbones and performance-critical applications.
Infiniband

Infiniband offers even higher performance than 10GbE. In addition to TCP/IP and UDP network connections used with Ethernet, Infiniband also supports shared memory communication. This allows Infiniband to work between systems via remote direct memory access (RDMA).

The use of RDMA allows Infiniband to move data directly between systems without the overhead of TCP/IP or socket connections. In turn, this reduces latency, which is critical to some applications.
Infiniband is most commonly used in High Performance Technical Computing (HPTC) applications which require high bandwidth, low latency and low overhead. Many supercomputing applications benefit from this, to the point that the best way to improve performance is by investing in Infiniband rather than faster processors or more memory.
RoCE

RDMA over Converged Ethernet (RoCE) implements Infiniband-style communications (including RDMA) over a 10GbE infrastructure. Given the cost improvements associated with the growing volume of 10GbE products, it is reasonable to expect wider usage of RDMA and RoCE in a wide range of systems and applications.

Each of these communication methods is fully-supported by Red Hat for use with Red Hat Enterprise Linux 6.
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