Explain the shared disk architecture.

A shared-disk parallel machine is one in which all processors can access the same disks with about the same performance, but are unable to access each other’s RAM. Shared disk has become an increasingly attractive approach in recent years, with the advent of Network Attached Storage devices (NAS), which allow a storage device on a network to be mounted by a set of nodes. One key advantage of shared-disk systems over shared-nothing is in usability, since DBAs of shared-disk systems do not have to consider partitioning tables across machines. Another feature of a shared-disk architecture is that the failure of a single DBMS processing node does not affect the other nodes’ ability to access the full database. This is in contrast to both shared-memory systems that fail as a unit, and shared-nothing systems that lose at least some data upon a node failure.
Because there is no partitioning of the data in a shared disk system, data can be copied into RAM and modified on multiple machines. Unlike shared-memory systems there is no place to coordinate this sharing of the data—each machine has its own local memory for locks and buffer pool pages. Hence there is a need to explicitly coordinate data sharing across the machines.