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.