Represents storage that is primarily in-memory, but has an associated file
which backs it onto more durable storage media. This allows us to use DRAM
to provide fast access to the storage region but with the ability to save
and restore from some file. It is recommended this file be kept on a fast
local disk (eg: NVMe), but it is not strictly required. Unlike simply using
mmap on the file, this ensures all the data is kept resident in-memory.
Represents storage that primarily exists in a file. This is best used in
combination with a DAX-aware filesystem on persistent memory to avoid page
cache pollution and interference. It can be used for volatile storage or
allow to resume from a clean shutdown.