Caching

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_target option, the original copy is left in place but marked as cached. With the promote_target option, the original copy is left unchanged and the new copy on the promote_target device is marked as cached.

To do writeback caching, set foreground_target and promote_target to the cache device, and background_target to the backing device. To do writearound caching, set foreground_target to the backing device and promote_target to the cache device.