HDFS is a distributed scalable filesystem written in Java for the Hadoop framework. HDFS is not a fully POSIX-compliant filesystem and does not support block storage, thus making it less usable than Ceph. The reliability of HDFS is a question for discussion as it's not a highly available filesystem. The single NameNode in HDFS is the primary reason for its single point of failure and performance bottleneck problems. It's more suitable to store a small quantity of big files rather than both small and big files.