7
As the pyramid shows, persistent memory provides latencies similar to memory,
measured in nanoseconds, while providing persistency. Block storage provides
persistency with latencies starting in the microseconds and increasing from there,
depending on the technology. Persistent memory is unique in its ability to act like both
memory and storage at the same time.
Program Complexity
Perhaps the most important point of our example is that the programmer still uses
the familiar get/put interfaces normally associated with key-value stores. The fact that
the data structures are in persistent memory is abstracted away by the high-level API
provided by libpmemkv. This principle of using the highest level of abstraction possible,
as long as it meets the application’s needs, will be a recurring theme throughout this
book. We start by introducing very high-level APIs; later chapters delve into the lowerlevel details for programmers who need them. At the lowest level, programming directly
to raw persistent memory requires detailed knowledge of things like hardware atomicity,
cache flushing, and transactions. High-level libraries like libpmemkv abstract away all
that complexity and provide much simpler, less error-prone interfaces.
Figure 1-3. The memory/storage hierarchy pyramid with estimated latencies
CHApter 1 IntroduCtIon to persIstent MeMory proGrAMMInG
As the pyramid shows, persistent memory provides latencies similar to memory,
measured in nanoseconds, while providing persistency. Block storage provides
persistency with latencies starting in the microseconds and increasing from there,
depending on the technology. Persistent memory is unique in its ability to act like both
memory and storage at the same time.
Program Complexity
Perhaps the most important point of our example is that the programmer still uses
the familiar get/put interfaces normally associated with key-value stores. The fact that
the data structures are in persistent memory is abstracted away by the high-level API
provided by libpmemkv. This principle of using the highest level of abstraction possible,
as long as it meets the application’s needs, will be a recurring theme throughout this
book. We start by introducing very high-level APIs; later chapters delve into the lowerlevel details for programmers who need them. At the lowest level, programming directly
to raw persistent memory requires detailed knowledge of things like hardware atomicity,
cache flushing, and transactions. High-level libraries like libpmemkv abstract away all
that complexity and provide much simpler, less error-prone interfaces.
Figure 1-3. The memory/storage hierarchy pyramid with estimated latencies
CHApter 1 IntroduCtIon to persIstent MeMory proGrAMMInG