G O O G L E F I L E S Y S T E M 陳 仕融 黃 振凱 林 佑恩 Z 1

OUTLINE  Introduction  Design Overview  System Interactions  Master Operation  Fault Tolerance and Diagnosis  Summary 2

INTRODUCTION 1.A scalable distributed file system 2.For large distributed data-intensive application 3.Fault tolerance, running on cheap hardware 4.High performance for large number of clients 3

INTRODUCTION: GFS IN USE  GFS is…  Widely deployed within Google  Provide hundreds of TBs of storage  For service, research and development use 4

INTRODUCTION  Observations of data usage in Google's application  Files are typically large; multiple GB files are common  Access pattern: most writes are appending while most reads are sequential(no need for caching)  Co-designing applications and file system benefits by increasing flexibility  Component failures are the norm 5

DESIGN OVERVIEW: ASSUMPTIONS  Cheap hardware often fail, and failure must be tolerated and recovered  Large files(GB~TB) must be managed efficiently  Large streaming reads, and small random reads  Large sequential append write  High sustained bandwidth, rather than response time 6

DESIGN OVERVIEW: INTERFACE  Synchronization & atomicity is encapsulated in the GFS library  Create  Delete  Open  Close  Read  Write  Snapshot  Record Append 7

DESIGN OVERVIEW: ENVIRONMENT  Linux machine running user-level applications  One master, multiple chunkservers  Files are divided into fixed-sized(64MB) chunks  Each chunk has a 64-bit handle(or identifier)  By default, each chunk is replicated 3 times  HeartBeat messages – check if server is alive  Client and chunkserver do not cache file data 8

DESIGN OVERVIEW: DATA FLOW 9

DESIGN OVERVIEW: MASTER  File system metadata  Metadata are small enough too keep in memory, which simplify the design and gain performance  File namespace  File to chunk mapping table  System-wide operation  A GFS cluster has only one master 10

SYSTEM INTERACTION  Goal: minimizes the master’s involvements in all operations  Leases and Mutation Order  Data Flow  Atomic Record Appends  Snapshot 11

SYSTEM INTERACTION  Leases and Mutation Order  Mutation: operations(EX: operations such as write or append) that changes the contents or metadata of chunks.  Mutations are performed at all the chunk’s replicas 12

SYSTEM INTERACTION  Lease( 租契 )  Master grants a chunk lease to one of the replica => primary  Primary than picks a serial order for all mutations to the chunk(without master’s intervention) 13

14 System interaction

SYSTEM INTERACTION  Data flow  fully utilize each machine’s network bandwidth : data is pushed linearly along a chain of chunkservers  Avoid network bottlenecks : each machine forwards the data to the closest network  Minimize the latency to push through all the data : pipelining the data transfer over TCP 15

SYSTEM INTERACTION  Atomic Record Appends  Client specifies only data. GFS appends the data to the file at least once atomically at an offset of GFS’s choosing and returns that offset to the client => guarantees replica is written at least once but does not guarantee all replicas are bytewise identical 16

SYSTEM INTERACTION  Snapshot  Makes a copy of a file or a directory  Snapshot implementation  Master receives a snapshot request  Master revokes leases on the chunks in the files it is about to snapshot  Master create a new copy of the chunk  Master logs the operation  Duplicates the metadata for the source file or directory tree  Newly created snapshot files point to same chunks as the source files 17

MASTER OPERATION  Namespace management and locking  GFS represents its namespace as a lookup table mapping full pathnames to metadata  Each node in the namespace tree has an associated read-write lock  Locking scheme  Require no write lock on the parent directory => allows concurrent mutations in the same directory  Read lock on the directory name to prevent the directory from being deleted, renamed, or snapshotted 18

MASTER OPERATION  Replica placement  Creation, Re-replication, Rebalancing  Chunk replicas are created for three reasons  Chunk creation  Re-replication  Rebalancing 19

MASTER OPERATION  Garbage collection  After a file is deleted, GFS does not immediately reclaim the available physical storage.  The file is renamed to a hidden name that includes the deletion timestamp  Remove hidden files for more than three days  The file can still be read and undeleted until it is removed  Orphaned chunks  Replica not known to the master is garbage  Regular background activities of the master 20

MASTER OPERATION  Stale replica detection  Master maintains a chunk version number for each chunk  Master removes stale replicas in its regular garbage collection  Client or the chunkserver always access up-to-date data 21

FAULT TOLERANCE AND DIAGNOSIS  High availability  Fast recovery  Chunk replication  Master replication  Master state is replicated for reliability – its operation logs and checkpoints are replicated on multiple machines  Shadow master – provides read-only access even the primary master is down 22

FAULT TOLERANCE AND DIAGNOSIS  Data integrity  Checksum  Each chunkserver independently verify the integrity of its own copy by maintaining checksums  A chunk is broken up into 64KB blocks. Each block has a corresponding 32 bits checksum  Chunkserver verifies the checksum. If mismatch, the requestor read from other replicas. Master clone correct replica and instructs the chunkserver to delete the false replica 23

FAULT TOLERANCE AND DIAGNOSIS  Diagnostic tools  Diagnostic logs that record many significant events and all RPC(Remote Procedure Call) requests and replies 24

SUMMARY  GFS is widely used within Google as the storage platform for research and development as well as production data processing Google File System is no doubt one of the crucial pusher that pushes Google to the top search engine of the world!! 25

THANKS FOR LISTENING 26