Big Data Computing Applications - Guide to Cloud Computing for Business and Technology Managers - page 449

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M M M M M M M M M M M M

Shared memory Shared disk Shared nothing

FIGURE 21.1
Parallel architectures.

1. In a shared memory system, all processors have direct access to all
memory via a shared bus. Typical examples are the common sym-
metric multiprocessor systems, where each processor core can access
the complete memory via the shared memory bus. To preserve the
abstraction, processor caches, buffering a subset of the data closer to
the processor for fast access, have to be kept consistent with special-
ized protocols. Because disks are typically accessed via the memory,
all processes also have access to all disks.

2. In a shared disk architecture, all processes have their own private
memory, but all disks are shared. A cluster of computers connected
to a SAN is a representative for this architecture.

3. In a shared nothing architecture, each processor has its private
memory and private disk. The data is distributed across all disks,
and each processor is responsible only for the data on its own con-
nected memory and disks. To operate on data that spans the differ-
ent memories or disks, the processors have to explicitly send data
to other processors. If a processor fails, data held by its memory
and disks is unavailable. Therefore, the shared nothing architecture
requires special considerations to prevent data loss.

When scaling out the system, the two main bottlenecks are typically the
bandwidth of the shared medium and the overhead of maintaining a con-
sistent view of the shared data in the presence of cache hierarchies. For
that reason, the shared nothing architecture is considered the most scalable
one, because it has no shared medium and no shared data. While it is often
argued that shared disk architectures have certain advantages for transac-
tion processing, the shared nothing is the undisputed architecture of choice
for analytical queries.
A shared-disk approach may have isolated processors, each with its
own memory, but the persistent storage on disk is still shared across the
system. These types of architectures are layered on top of SMP machines.
While there may be applications that are suited to this approach, there are