According to the storage engine documentation:
When a write comes in the new points are serialized, compressed using Snappy, and written to a [Write-Ahead-Log] file. The file is fsync’d and the data is added to an in-memory index before a success is returned.
We try to do things in a sane order so that we can recover from sudden failures, and while it is neither tested nor supported, data should not be lost in your scenario and backups using filesystem snapshots should generally be OK. The approach might come with some cost in startup time when restoring the system, since the WAL files will have to be loaded into memory, but you will have to decide whether that is an acceptable trade-off.
