The Google File System

Presenters: Rezan Amiri Sahar Delroshan

Azad university of Kurdistan

Outline

• Distributed File systems Overview

• GFS (Google File System)– Motivations– Assumptions– Architecture– Algorithms– conclusions

Azad university of Kurdistan 2 The Google File System

Distributed File systems

• Sneaker net is the process of sharing files by copying them onto floppy disks, physically carrying it to another computer and copying it again (1980S)

• a file system that joins together the

file systems of individual machines in

network

• Files are stored (distributed) on

different machines in a computer

network but are accessible from all

machines.

• called network file systems

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Examples of Distributed File System

• Distributed file systems– Andrew File System (AFS)

– Network File System (NFS)

– Apple Filing Protocol (AFP)

• Distributed fault tolerant file

systems– Coda

– Distributed File System (Microsoft)

(DFS(Azad university of Kurdistan 4 The Google File System

Motivation• Who doesn’t know about Google?

– the accuracy is dependent on how the algorithm is designed.

– Lary Page & Sergey Brin 1998• Google is beyond the searching

– Google Video– Gmail– Google Map– Google Earth,…

• Google OperationsAzad university of Kurdistan 5 The Google File System

Motivation• Redundant storage of massive amounts of

data on cheap and unreliable computers• problems caused by (OS bugs , human

errors , …)

• Why not use an existing file system?– Google’s problems are different from anyone

else’s

– Different workload and design priorities

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Assumptions• High component failure rates

– Inexpensive commodity components fail all the time

• Workloads – large streaming read, Small random

reads , Writes• efficiently implement well-defined

semantics for multiple clients that concurrently append

to the same file• High sustained bandwidth is more

important than low latency Azad university of Kurdistan 7 The Google File System

GFS Design Decisions

• Files stored as chunks– Fixed size (64MB)

• Reliability through replication– Each chunk replicated across 3+ chunk

servers

• Single master to coordinate access,

keep metadata

• Add snapshot and record append

operationsAzad university of Kurdistan 8 The Google File System

GFS Architecture

• 1 master• many chunks server • Files is divided into

large chunks• chunks are replicated across the system

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Chunks and Chunk Servers

• Similar to block in file systems– Size is always 64 MB– Lazy allocation to avoid fragmentation

• Reduce the need to contact to master• Reduce network overhead

– Client consistently maintain TCP connection with chunk server

• Reduce size of metadata in memory– Allow to keep metadata in memory

• Problem :” hot spot “ with small file

Azad university of Kurdistan 10 The Google File System

Metadata• Global metadata is stored on the master

– File and chunk namespaces– Mapping from files to chunks– Locations of each chunk’s replicas

• All in memory (64 bytes / chunk)– Fast– Easily accessible

• operation log– Persistent on local disk– Replicated – Checkpoints for faster recovery

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Master’s Responsibilities

• Master is a single process running on a

separate machine that stores all

metadata

• Periodic communication with chunk

servers

• Namespace management and locking– It allows concurrent mutations in the same

directory

• creation, re-replication, rebalancingAzad university of Kurdistan 12 The Google File System

Master’s Responsibilities

• The master re-replicates a chunk as soon as the number of available replicas falls below a user-specified goal :– a chunk server becomes unavailable

• Chunk re-replicated is prioritized– how far it is from its replication goal– Re-replicate for live files

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Master’s Responsibilities

• Replica placement– Improve reliability, availability– network bandwidth utilization

• Garbage Collection– simpler, more reliable than traditional file

delete– master logs the deletion, renames the file

to a hidden name• Lease mechanism

– Help reduce the overhead at masterAzad university of Kurdistan 14 The Google File System

Read Algorithm

1 .Application originates the read request2. GFS client translates request and sends it to master3. Master responds with chunk handle and replica Locations

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Read Algorithm

4.Client picks a location and sends the request5. Chunk server sends requested data to the client6. Client forwards the data to the application

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Write Algorithm

4.Client pushes write data to all locations.Data is stored in chunk server's internal buffers

1,2,3 as like as read algorithm

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Write Algorithm

5.Client sends write command to primary6. Primary determines serial order for data instances in its buffer and writes the instances in that order to the Chunk7. Primary sends the serial order to the secondary's and tells them to perform the write18 The Google File SystemAzad university of Kurdistan

Write Algorithm

9. Primary responds back to the client

8.Secondaries respond back to primary

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Leases and mutation order

The Google File System20Azad university of Kurdistan

Append Algorithm• GFS appends it to the file atomically at least once

- Client determine data- GFS picks the offset

1,2,3 ,4 as like as write algorithm5. Primary checks if record fits in specified chunk6. If the record does not fit: 1. Pads the chunk 2. Tells secondary to do the same 3. Informs client 4. Client then retries the append with the next chunk 21 The Google File SystemAzad university of Kurdistan

Append Algorithm

7. If record fits, then the primary:

1. Appends the record

2.Tells secondary's to do the same 3.Receives responses from secondary's 4. And sends final response to the client

• snapshot- Fast Replicate from all system

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Fault Tolerance• High availability

– Fast recovery– chunk replication

• default: 3 replicas– replication

• Shadow master• Data integrity

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Performance Test

Conclusion

• GFS demonstrates how to support

large-scale processing workloads on

commodity hardware

• Performance

• Scalability

• Fault-tolerance

• Minimize master involvement

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Strong point

• Single master simplify design fast simple failure

handling

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weakness

• Replication– can be out of sync– Space overhead– Consistency among replica(what if

replica has different length)

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References

• Sanjay Ghemawat , Howard Gobioff , and Shun- Tak Leung, The Google File System, ACM Symposium on Operating Systems Principles, 2003

• Naushad UzZaman, Survey on Google File System, CSC 456 (Operating Systems), 2007

• Wikipedia Contributors, Google File System, Wikipedia -The Free Encyclopedia, 2010

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Questions?

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