Google file system

Editor's Notes

• #6: GFS – single master, multiple chunkservers, multiple client. Files- divided into chunks, chunks- immutable and globally unique 64 bit chunk handle. Stored in multiple chunkservers, master- contains metadata includes the namespace, access control information, mapping of file to chunks and current location of chunks
• #7: Single Master- can make sophisticated chunk replacement and replication decisions using global knowledge. Read example Chunk Size – 64 MB, advantages – reduces client-master interation, client more likely to perform many operations on given chunk, reduces metadata size. Metadata – stores file and chunk namespaces, mapping from files to chunks, location to chunk’s relica, metadata stored in memory to do fast operations, chunk location – does not keep a record, polls at startup, monitor by sending heartbeat messages,operation log- contains a history of critical metadata changes. Guarantee- application mutation on same order to all the replicas , using chunk version numbers to detect any replica Consistent – all replicas have the same data, defined – consistent – defined and client can see what the mutation has written
• #8: Mutation – operation that changes the content of metadata Data flow – bandwidth – data is [pushed linearly along the server, avoid bottlenecks and high-latency links- each machine forwards the data to closest possible, latency min – pipelining the data transfer over TCP connections. Record append – client specifies the data, GFS appends automatically, same way as control flow Snapshots – makes a copy of file or ‘directory tree’ minimizing any interruption with ongoing mutations
• #9: Master – executes all namespace operations, manages chunk replicas, Namespace – GFS logically represent its namespace as a look up table mapping full path names to metadata. Replica placement - 1) maximise data reliability and availability, and 2) maximum bandwidth utilization Creation, re-replication – replicas on severs with below average disk utilization, limit recent creation on each chunk server, spread replicas of a chunk across racks Garbage collection – after deletion, file renamed to a hidden file, deleted after 3 days, orphaned chunks, State replica detection – chunkserver failure missing mutation while it is down, master assigns – chunk server numbers to distinguish 
• #10: Fast recovery – mast and chunk server designed such that they restore their data and start in two seconds Chunk replication – discussed earlier Master replication – operations log and checkpoints are replicated on multiple machines, shadow masters – provide read-only access Data integrity – uses checksumming to detect corruption of stored data, we can recover from corruption using replicas, but it is impractical Diagnostic tools – generate diagnostic logs that record many significant events. The RPC logs include the exact requests and responsessent on the wire, except for the file data being read or written.
• #12: The two clusters have similar numbers of files, though B has a larger proportion of dead files, namely files which were deleted or replaced by a new version but whose storage have not yet been reclaimed. It also has more chunks because its files tend to be larger 
• #14: Read returns no data in Y b’coz applications in production system use file as producer-consumer queues cluster Y sees a much higher percentage of large record appends than cluster X does becauseour production systems, which use cluster Y, are more aggressively tuned for GFS