SSD based InnoDB buffer pool extension

In one of the practical cloud MariaDB setups, a server node accesses its datadir over the network, but also has a fast local SSD storage for temporary data. The content of such temporary storage is lost when the server container is destroyed.

It could make sense to use this ephemeral fast local storage (SSD) as an extension of the portion of InnoDB buffer pool (DRAM) that caches persistent data pages. This cache would be separate from the persistent storage of data files and ib_logfile0.

Such a local cache would avoid page reads and writes on slow network or HDD storage. On the persistent store, only ib_logfile0 would be written, until a write-back is needed for other reasons.

We can simply treat the combination of the buffer pool and the local disk as one unit. Persistent pages could be freely moved between the local disk and the buffer pool. We must keep track of which pages in this combined "virtual buffer pool" are dirty, that is, will have to be written back to the persistent storage. Any durability guarantees would only apply to the persistent storage.