GFS - Google File System

The Google File System Tut Chi Io

Design Overview – Assumption Inexpensive commodity hardware Large files: Multi-GB Workloads Large streaming reads Small random reads Large, sequential appends Concurrent append to the same file High Throughput > Low Latency

Design Overview – Interface Create Delete Open Close Read Write Snapshot Record Append

Design Overview – Architecture Single master, multiple chunk servers, multiple clients User-level process running on commodity Linux machine GFS client code linked into each client application to communicate File -> 64MB chunks -> Linux files on local disks of chunk servers replicated on multiple chunk servers (3r) Cache metadata but not chunk on clients

Design Overview – Single Master Why centralization? Simplicity! Global knowledge is needed for Chunk placement Replication decisions

Design Overview – Chunk Size 64MB – Much Larger than ordinary, why? Advantages Reduce client-master interaction Reduce network overhead Reduce the size of the metadata Disadvantages Internal fragmentation Solution: lazy space allocation Hot Spots – many clients accessing a 1-chunk file, e.g. executables Solution: Higher replication factor Stagger application start times Client-to-client communication

Design Overview – Metadata File & chunk namespaces In master’s memory In master’s and chunk servers’ storage File-chunk mapping In master’s memory In master’s and chunk servers’ storage Location of chunk replicas In master’s memory Ask chunk servers when Master starts Chunk server joins the cluster If persistent, master and chunk servers must be in sync

Design Overview – Metadata – In-memory DS Why in-memory data structure for the master? Fast! For GC and LB Will it pose a limit on the number of chunks -> total capacity? No, a 64MB chunk needs less than 64B metadata (640TB needs less than 640MB) Most chunks are full Prefix compression on file names

Design Overview – Metadata – Log The only persistent record of metadata Defines the order of concurrent operations Critical Replicated on multiple remote machines Respond to client only when log locally and remotely Fast recovery by using checkpoints Use a compact B-tree like form directly mapping into memory Switch to a new log, Create new checkpoints in a separate threads

Design Overview – Consistency Model Consistent All clients will see the same data, regardless of which replicas they read from Defined Consistent, and clients will see what the mutation writes in its entirety

Design Overview – Consistency Model After a sequence of success, a region is guaranteed to be defined Same order on all replicas Chunk version number to detect stale replicas Client cache stale chunk locations? Limited by cache entry’s timeout Most files are append-only A Stale replica return a premature end of chunk

System Interactions – Lease Minimized management overhead Granted by the master to one of the replicas to become the primary Primary picks a serial order of mutation and all replicas follow 60 seconds timeout, can be extended Can be revoked

System Interactions – Mutation Order Current lease holder? identity of primary location of replicas (cached by client) 3a. data 3b. data 3c. data Write request Primary assign s/n to mutations Applies it Forward write request Operation completed Operation completed Operation completed or Error report

System Interactions – Data Flow Decouple data flow and control flow Control flow Master -> Primary -> Secondaries Data flow Carefully picked chain of chunk servers Forward to the closest first Distances estimated from IP addresses Linear (not tree), to fully utilize outbound bandwidth (not divided among recipients) Pipelining, to exploit full-duplex links Time to transfer B bytes to R replicas = B/T + RL T: network throughput, L: latency

System Interactions – Atomic Record Append Concurrent appends are serializable Client specifies only data GFS appends at least once atomically Return the offset to the client Heavily used by Google to use files as multiple-producer/single-consumer queues Merged results from many different clients On failures, the client retries the operation Data are defined, intervening regions are inconsistent A Reader can identify and discard extra padding and record fragments using the checksums

System Interactions – Snapshot Makes a copy of a file or a directory tree almost instantaneously Use copy-on-write Steps Revokes lease Logs operations to disk Duplicates metadata, pointing to the same chunks Create real duplicate locally Disks are 3 times as fast as 100 Mb Ethernet links

Master Operation – Namespace Management No per-directory data structure No support for alias Lock over regions of namespace to ensure serialization Lookup table mapping full pathnames to metadata Prefix compression -> In-Memory

Master Operation – Namespace Locking Each node (file/directory) has a read-write lock Scenario: prevent /home/user/foo from being created while /home/user is being snapshotted to /save/user Snapshot Read locks on /home, /save Write locks on /home/user, /save/user Create Read locks on /home, /home/user Write lock on /home/user/foo

Master Operation – Policies New chunks creation policy New replicas on below-average disk utilization Limit # of “recent” creations on each chun server Spread replicas of a chunk across racks Re-replication priority Far from replication goal first Chunk that is blocking client first Live files first (rather than deleted) Rebalance replicas periodically

Master Operation – Garbage Collection Lazy reclamation Logs deletion immediately Rename to a hidden name Remove 3 days later Undelete by renaming back Regular scan for orphaned chunks Not garbage: All references to chunks: file-chunk mapping All chunk replicas: Linux files under designated directory on each chunk server Erase metadata HeartBeat message to tell chunk servers to delete chunks

Master Operation – Garbage Collection Advantages Simple & reliable Chunk creation may failed Deletion messages may be lost Uniform and dependable way to clean up unuseful replicas Done in batches and the cost is amortized Done when the master is relatively free Safety net against accidental, irreversible deletion

Master Operation – Garbage Collection Disadvantage Hard to fine tune when storage is tight Solution Delete twice explicitly -> expedite storage reclamation Different policies for different parts of the namespace Stale Replica Detection Master maintains a chunk version number

Fault Tolerance – High Availability Fast Recovery Restore state and start in seconds Do not distinguish normal and abnormal termination Chunk Replication Different replication levels for different parts of the file namespace Keep each chunk fully replicated as chunk servers go offline or detect corrupted replicas through checksum verification

Fault Tolerance – High Availability Master Replication Log & checkpoints are replicated Master failures? Monitoring infrastructure outside GFS starts a new master process “Shadow” masters Read-only access to the file system when the primary master is down Enhance read availability Reads a replica of the growing operation log

Fault Tolerance – Data Integrity Use checksums to detect data corruption A chunk(64MB) is broken up into 64KB blocks with 32-bit checksum Chunk server verifies the checksum before returning, no error propagation Record append Incrementally update the checksum for the last block, error will be detected when read Random write Read and verify the first and last block first Perform write, compute new checksums

Conclusion GFS supports large-scale data processing using commodity hardware Reexamine traditional file system assumption based on application workload and technological environment Treat component failures as the norm rather than the exception Optimize for huge files that are mostly appended Relax the stand file system interface

Conclusion Fault tolerance Constant monitoring Replicating crucial data Fast and automatic recovery Checksumming to detect data corruption at the disk or IDE subsystem level High aggregate throughput Decouple control and data transfer Minimize operations by large chunk size and by chunk lease

Reference Sanjay Ghemawat, Howard Gobioff, and Shun-Tak Leung, “The Google File System”

GFS - Google File System

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GFS - Google File System