Ceph » rbd

After filling the ssd cache to capacity and maintaining the load (so that the cache is constantly retiring old and generating new log entries), I have observed that the head (m_first_valid_entry) and tail (m_first_free_entry) pointers can get corrupted. Since the cache is at capacity, they are expected to cycle through the log space very close to each other, with the invariant that head > tail (head - tail = 8-12K, the rest is allocated). However, the tail pointer sometimes "jumps" the head pointer, violating the invariant and making the full cache appear almost empty (only 8-12K used).