Is data only written to disk when mem_high_wat is hit?
From the documentation:
At this point a background job is scheduled to ensure items are migrated to disk and the memory is then available for other membase items.
In the documentation, first the word migration is used, which sounds like it may mean either persistence or eviction and than later, the word eviction is used.
Does this mean that data is only persisted to disk when mem_high_wat is hit and any crash on a unloaded node will have full data loss?
We will also eject data as soon as it's persisted iff it's for an inactive (e.g. replica) vbucket if we're above our low watermark for memory.
Seems like it's persisted regardless. You might want to change from using the word migration and just say eviction from memory. Migration clearly sounds like your moving it to the hard drive.