The figure below shows the configuration of HDFS tiered storage.

All 4 blocks will be written to RAM_DISK

If it is a copy other than the first copy that fails to write, try to write to the storage type specified by "Alternative storage policy for copy"

The first block copy will be written to RAM_DISK, the rest of the replicas are written to DISK

If the first block copy is written to RAM_If DISK fails, try to write to the storage type specified by "Alternative Storage Policy"