How would you design a distributed file system like HDFS or GFS?

Architecture: master (namespace, metadata, chunk locations) and chunk servers (store data chunks). Files split into chunks (64-128MB), replicated across chunk servers (typically 3 replicas). Write: client gets chunk servers from master, writes to primary which replicates. Read: client gets chunk locations, reads from nearest replica. Master: single (with hot standby), handles metadata only, uses operation log + checkpoints. Chunk servers: report chunk lists to master, heartbeat for failure detection. Consistency: lease-based write ordering, atomic record append. Fault tolerance: re-replicate on failure detection. GC: lazy deletion, background cleanup.