When an extent has multiple copies on different devices, some of those copies may be marked as cached. Buckets containing only cached data are discarded as needed by the allocator in LRU order.
When data is moved from one device to another according to the
background_targetoption, the original copy is left in place but marked as cached. With the
promote_targetoption, the original copy is left unchanged and the new copy on the
promote_targetdevice is marked as cached.
To do writeback caching, set
promote_target to the cache device, and
background_target to the
backing device. To do writearound caching, set
the backing device and
promote_target to the cache device.