Some devices may be considered to be more reliable than others. For example, we might have a filesystem composed of a hardware RAID array and several NVME flash devices, to be used as cache. We can set replicas=2 so that losing any of the NVME flash devices will not cause us to lose data, and then additionally we can set durability=2 for the hardware RAID device to tell bcachefs that we don’t need extra replicas for data on that device - data on that device will count as two replicas, not just one.

The durability option can also be used for writethrough caching: by setting durability=0 for a device, it can be used as a cache and only as a cache - bcachefs won’t consider copies on that device to count towards the number of replicas we’re supposed to keep.