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1833 Parallel Database Architectures
There are several architectural models for parallel machines Among the most prominent ones are those in Figure 188 (in the gure, M denotes memory, P denotes a processor, and disks are shown as cylinders): Shared memory All the processors share a common memory (Figure 188a) Shared disk All the processors share a common set of disks (Figure 188b) Shared-disk systems are sometimes called clusters Shared nothing The processors share neither a common memory nor common disk (Figure 188c) Hierarchical This model is a hybrid of the preceding three architectures (Figure 188d) In Sections 18331 through 18334, we elaborate on each of these models Techniques used to speed up transaction processing on data-server systems, such as data and lock caching and lock de-escalation, outlined in Section 1822, can also be used in shared-disk parallel databases as well as in shared-nothing parallel databases In fact, they are very important for ef cient transaction processing in such systems
P M P P P P
M M M M M
P P P P P
(a) shared memory
M P P M P P M P M M P P P P P M
(b) shared disk
P P P P P
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(c) shared nothing
(d) hierarchical
Parallel database architectures
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
VI Database System Architecture
18 Database System Architecture
The McGraw Hill Companies, 2001
18
Database System Architectures
18331 Shared Memory
In a shared-memory architecture, the processors and disks have access to a common memory, typically via a bus or through an interconnection network The bene t of shared memory is extremely ef cient communication between processors data in shared memory can be accessed by any processor without being moved with software A processor can send messages to other processors much faster by using memory writes (which usually take less than a microsecond) than by sending a message through a communication mechanism The downside of shared-memory machines is that the architecture is not scalable beyond 32 or 64 processors because the bus or the interconnection network becomes a bottleneck (since it is shared by all processors) Adding more processors does not help after a point, since the processors will spend most of their time waiting for their turn on the bus to access memory Shared-memory architectures usually have large memory caches at each processor, so that referencing of the shared memory is avoided whenever possible However, at least some of the data will not be in the cache, and accesses will have to go to the shared memory Moreover, the caches need to be kept coherent; that is, if a processor performs a write to a memory location, the data in that memory location should be either updated at or removed from any processor where the data is cached Maintaining cache-coherency becomes an increasing overhead with increasing number of processors Consequently, shared-memory machines are not capable of scaling up beyond a point; current shared-memory machines cannot support more than 64 processors
18332 Shared Disk
In the shared-disk model, all processors can access all disks directly via an interconnection network, but the processors have private memories There are two advantages of this architecture over a shared-memory architecture First, since each processor has its own memory, the memory bus is not a bottleneck Second, it offers a cheap way to provide a degree of fault tolerance: If a processor (or its memory) fails, the other processors can take over its tasks, since the database is resident on disks that are accessible from all processors We can make the disk subsystem itself fault tolerant by using a RAID architecture, as described in 11 The shared-disk architecture has found acceptance in many applications The main problem with a shared-disk system is again scalability Although the memory bus is no longer a bottleneck, the interconnection to the disk subsystem is now a bottleneck; it is particularly so in a situation where the database makes a large number of accesses to disks Compared to shared-memory systems, shared-disk systems can scale to a somewhat larger number of processors, but communication across processors is slower (up to a few milliseconds in the absence of special-purpose hardware for communication), since it has to go through a communication network DEC clusters running Rdb were one of the early commercial users of the shareddisk database architecture (Rdb is now owned by Oracle, and is called Oracle Rdb Digital Equipment Corporation (DEC) is now owned by Compaq)
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