This may be a naive question, but one I haven’t seen addressed is: as a user, why would I want a new storage engine using these alternatives (RocksDB, LMDB, etc) instead of the existing ones, and what features would it offer me over using the existing engine? There are a lot of suggestions for adding different key/value engines onto FoundationDB in this thread, but there are no insights as to why they are good additions, or what gaps they fill, or excel at, that the current storage engine does suboptimally at.
It is worth noting that FoundationDB 6.1 will apparently feature a new “Redwood Storage Engine”, which will be described in the upcoming Summit 2018 talk “The Future of FoundationDB Storage Engines”. This has already been merged into master. This new storage engine seems like it will have some major distinct advantages that require more changes than just coupling an existing storage engine onto the bottom of FDB.
In addition to higher write throughput, this new design will have a smaller space footprint via key prefix compression and a configurable ability to enable long running read transactions by trading off a reasonable amount of disk space to persist old data versions on disk.
This seems like a great advancement and being able to plan my design around long-running read transactions being available is exciting. An interesting question would be what Redwood could further have improved about it, too (including a new low-level storage/tree layer, perhaps)
While we’re at it and I’m throwing out naive ideas here, it would be interesting to think about a persistent-memory backend for FoundationDB. Intel is soon going to ship Optane DIMMs with more widespread availability in their new Cascade Lake platforms. pmem support is also actively available in QEMU, so it can be programmed against, just not with realistic performance numbers. But having a storage engine designed for highly-dense persistent DIMMs (~7TB with lower latency than NVMe, per machine) would be very interesting to experiment with, and Optane seems like it could be very attractive for a lot of workloads.