In Design Data Intensive Applications,
If all you need is to scale to higher load, the simplest approach is to buy a more powerful machine (sometimes called vertical scaling or scaling up). Many CPUs, many RAM chips, and many disks can be joined together under one operating system, and a fast interconnect allows any CPU to access any part of the memory or disk. In this kind of shared-memory architecture, all the components can be treated as a single machine .
The problem with a shared-memory approach is that the cost grows faster than linearly: a machine with twice as many CPUs, twice as much RAM, and twice as much disk capacity as another typically costs significantly more than twice as much. And due to bottlenecks, a machine twice the size cannot necessarily handle twice the load.
A shared-memory architecture may offer limited fault tolerance—high-end machines have hot-swappable components (you can replace disks, memory modules, and even CPUs without shutting down the machines)—but it is definitely limited to a single geographic location.
Another approach is the shared-disk architecture, which uses several machines with independent CPUs and RAM, but stores data on an array of disks that is shared between the machines, which are connected via a fast network. ii This architecture is used for some data warehousing workloads, but contention and the overhead of locking limit the scalability of the shared-disk approach .
By contrast, shared-nothing architectures  (sometimes called horizontal scaling or scaling out) have gained a lot of popularity. In this approach, each machine or virtual machine running the database software is called a node. Each node uses its CPUs, RAM, and disks independently. Any coordination between nodes is done at the software level, using a conventional network.
Is shared disk architecture scale-up or scale out or both or neither?
The architectures uses multiple machines, since "several machines with independent CPUs and RAM, but stores data on an array of disks that is shared between the machines".
But I am not sure if the architecture is scaling out, because the machines share disks, and the book doesn't mention scaling out until shared nothing architecture.