The Google File System

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1 The Google File System Sanjay Ghemawat, Howard Gobioff, and Shun-Tak Leung SOSP 2003 presented by Kun Suo

2 Outline GFS Background, Concepts and Key words Example of GFS Operations Some optimizations in GFS Evaluation Conclusion

3 Motivation

4 What is the GFS? Google File System is a scalable distributed file system for large distributed data-intensive applications, which runs on inexpensive commodity hardware and provides fault tolerance, high performance to a large number of clients. GFS shares many of the same goals as previous distributed file systems such as performance, scalability, reliability, and availability

5 GFS Assumptions Hardware: The system is built from many inexpensive commodity components that often fail File: The system stores a modest number of large files Workloads characteristics: - Large streaming reads - Small random reads. - Many large, sequential writes that append data to files Client: the system must efficiently implement for multiple clients that concurrently append to the same file. Target: High sustained bandwidth is more important than low latency

6 Interface of GFS GFS provides a familiar file system interface: support the usual operations to create, delete, open, close, read, and write files. GFS supports snapshot and record append operations - Producer-Consumer queues - Many-way merging

7 Architecture of GFS GFS components: - One single master - Multiple Clients - Multiple GFS chunkserver

8 Chunk Size Chunksize is set as 64MB Pro: - Less interoperation between client and master node - Keep TCP long connection, less network overhead - Less meta data on master node Con: - Small file - Too many clients visit the same file, hot spots

9 Metadata Three types of metadata: - (1) File and chunk namespaces - (2) Mapping from files to chunks - (3) Locations of each chunk s replicas All metadata is kept in master memory (performance) - Fast - Easily accessible (1) & (2) are kept persistent by logging (Reliability); (3) will be updated periodically

10 Master Node Metadata storage Namespace management Periodically communicate with chunkservers Chunk operation: create, re-replicate, delete, garbage collection, load balance, etc.

11 System Interaction (1) Mutation (2) Lease Minimize management overhead at the master

12 Mutation Mutation = write or append to the contents or metadata of a chunk - Must be done for all replicas (Consistency) Lease - Master picks one replica as primary; gives it a lease for mutations for all replicas Purpose - Data flow decoupled from control flow - Minimize master involvement

13 Outline GFS Background, Concepts and Key words (Question) Example of GFS Operations Some optimizations in GFS Evaluation Conclusion

14 Question [1] its design has been driven by key observations of our application workloads and technological environment, What are the workload and technology characteristics GFS assumed in its design and what are their corresponding design choices? > GFS design assumptions and target workload

15 GFS Assumptions Hardware: The system is built from many inexpensive commodity components that often fail File: The system stores a modest number of large files Workloads characteristics: - Large streaming reads - Small random reads. - Many large, sequential writes that append data to files Client: the system must efficiently implement for multiple clients that concurrently append to the same file. Target: High sustained bandwidth is more important than low latency

16 Question [2] while caching data blocks in the client loses its appeal. GFS does not cache file data. Why does this design choice not lead to performance loss? What benefit does this choice have? (1) stream through huge files (a) Simply design of GFS client (2) working sets too large server (b)eliminating cache coherence issues, challenging Client caches offer little benefit. However, clients still cache metadata for future access.

17 Question [3] Small files must be supported, but we need not optimize for them. Why? (a) GFS is designed to store millions of large files, each typically 100 MB or larger in size Large and small files exist in almost every systems. (b) The chunkservers storing chunks which belong to small files may become hot spots if many clients are accessing the same file. In practice, hot spots have not been a major issue because our applications mostly read large multi-chunk files sequentially. (c) One of disadvantages of GFS

18 Outline GFS Background, Concepts and Key words Example of GFS Operations Some optimizations in GFS Evaluation Conclusion

19 Read in GFS Application 1, Application originates the read request data 6 Client 5 data from file 1 file name, byte range file name, chunk index chunk handle byte range chunk handle replica location Master 2, GFS client translates request and sends it to master 3, Master responds with chunk handle and replica locations Chunk Chunk Chunk

20 Read in GFS data 6 Application Client 5 data from file 1 file name, byte range file name, chunk index chunk handle byte range chunk handle replica location Master 4, Client picks a location and sends the request 5, Chunkserver sends requested data to the client 6, Client forwards the data to the application Chunk Chunk Chunk

21 Write on GFS Application 9 Client 4 1 file name, byte range Master 1. Application originates the request 2. GFS client translates request and sends it to master 3. Master responds with chunk handle and replica locations Chunk replica 6 Chunk (Primary) 6 Chunk replica 7 7

22 Write on GFS Application 9 Client 1 file name, byte range 2 Master 4, Client pushes write data to all locations. Data is stored in chunkserver s internal buffers , Client sends write command to primary Chunk replica 6 Chunk (Primary) 6 Chunk replica 7 7

23 Write on GFS Application 9 Client 1 file name, byte range 2 Master 6, Primary determines serial order for data instances in its buffer and writes the instances in that order to the chunk Primary sends the serial order to the secondaries and tells them to perform the write Chunk replica 6 Chunk (Primary) 6 Chunk replica 7 7

24 Write on GFS Application 9 Client 4 1 file name, byte range 7, Secondaries respond back to primary Master 8, Primary responds back to the client 9, Client responds to applications Chunk replica 6 Chunk (Primary) 6 Chunk replica 7 7

25 Append on GFS In a traditional write, the client specifies the offset at which data is to be written. Append is same as write, but no offset. GFS picks the offset and works for concurrent writers difference

26 Outline GFS Background, Concepts and Key words Example of GFS Operations (Question) Some optimizations in GFS Evaluation Conclusion

27 Question [4] Clients interact with the master for metadata operations, but all data-bearing communication goes directly to the chunkservers. How does this design help improve the system s performance? Potential bottleneck minimize clients involvement in reads and writes with the master node

28 Question [5] A GFS cluster consists of a single master. What s benefit of having only a single master? What s its potential performance risk? How does GFS minimize such a risk? 1, Simplify Design 2, Potential bottleneck 3, Minimize clients involvement in reads and writes with the master node

29 Question [6] Each chunk replica is stored as a plain Linux file on a chunkserver and is extended only as needed. How does GFS collaborate with chunkserver s local file system to store file chunks? What s lazy space allocation and what s its benefit? GFS is composed of many servers Each server is typically a commodity Linux machine running a user-level server process. The file in GFS is finally stored in local server as regular Linux file

30 Question [6] Each chunk replica is stored as a plain Linux file on a chunkserver and is extended only as needed. How does GFS collaborate with chunkserver s local file system to store file chunks? What s lazy space allocation and what s its benefit? with help of local file system

31 Question [6] Each chunk replica is stored as a plain Linux file on a chunkserver and is extended only as needed. How does GFS collaborate with chunkserver s local file system to store file chunks? What s lazy space allocation and what s its benefit? Lazy allocation simply means not allocating a resource until it is actually needed. Benefits: Lazy space allocation avoids wasting space due to internal fragmentation, perhaps the greatest objection against such a large chunksize.

32 Question [7] On the other hand, a large chunks size, even with lazy space allocation, has its disadvantages. Give an example disadvantage. A small file consists of a small number of chunks, perhaps just one. The chunkservers storing those chunks may become hot spots if many clients are accessing the same file. In practice, hot spots did develop when GFS was first used by a batch-queue system. The few chunkservers storing an executable problem were overloaded by hundreds of simultaneous requests. Fixed by storing such executables with a higher replication factor and by making the batchqueue system stagger application start times.

33 Question [7] On the other hand, a large chunks size, even with lazy space allocation, has its disadvantages. Give an example disadvantage. [Example] hot spot for small files Chunk

34 Question [8] One potential concern for this memory-only approach is that the number of chunks and hence the capacity of the whole system is limited by how much memory the master has. Why is GFS s master able to keep the metadata in memory? Chunk size (64MB) > less than 64 bytes Metadata, small enough

35 Question [9] We use leases to maintain a consistent mutation order across replicas. Could you show a scenario where unexpected result may appear if the lease mechanism is not implemented? Also explain how leases help address the problem? without lease primary order order: A, B, C order: A, C, B non-primary order order: B, A, C

36 Question [9] We use leases to maintain a consistent mutation order across replicas. Could you show a scenario where unexpected result may appear if the lease mechanism is not implemented? Also explain how leases help address the problem? primary order order: A, B, C follow it with lease order: A, B, C non-primary order order: A, B, C

37 Question [9] We use leases to maintain a consistent mutation order across replicas. Could you show a scenario where unexpected result may appear if the lease mechanism is not implemented? Also explain how leases help address the problem? Lease: keep mutation order Secondary replicas follows primary replica

38 Outline GFS Background, Concepts and Key words Example of GFS Operations Some optimizations in GFS Evaluation Conclusion

39 Some Optimizations on GFS Snapshot Fault tolerance Relaxed Consistency Model

40 Snapshot A snapshot is a copy of a system at a moment at low cost Snapshot is implemented based on standard copy-onwrite Why we use snapshot? - To quickly create branch copies of huge data sets (Performance) - A quick data access for end users (Performance) - Changes committed or rolled-back easily (Reliability)

41 Fault Tolerance High availability - Fast recovery - Master and Chunkservers: failed, restart in a few seconds Chunk replication - Each chunk is replicated on multiple chunkservers on different tracks. Users can specify different levels for different parts of the file namespace. - default: 3 replicas Shadow masters - Checksum every 64KB block in each chunk

42 Relaxed Consistency Model Relying on appends rather than overwrites, checkpointing, and writing self-validating, self-identifying records - far more efficient and resilient to Apps Many writers concurrently append to a file for merged results or as a producer-consumer queue - simple, efficient - Google apps live with it

43 Outline GFS Background, Concepts and Key words Example of GFS Operations Some optimizations in GFS (Question) Evaluation Conclusion

44 Question [10] When the master creates a chunk, it chooses where to place the initially empty replicas. What are criteria for choosing where to place the initially empty replicas? new 1, place new replicas on chunkservers with below-average diskspace utilization (balance) 2, limit the number of recent creations on each chunkserver (imminent heavy write soon) 3, spread replicas of a chunkacross racks (reliability)

45 Question [11] The master re-replicates a chunk as soon as the number of available replicas falls below a user-specified goal. When a new chunkserver is added into the system, the master mostly uses chunk rebalancing rather than using new chunks to fill up it. Why? Heavy I/O flow, bad :( Put eggs in one basket, not safe 2, limit the number of recent creations on each chunkserver (imminent heavy write soon) 3, spread replicas of a chunkacross racks (reliability)

46 Question [12] After a file is deleted, GFS does not immediately reclaim the available physical storage. It does so only lazily during regular garbage collection at both the file and chunk levels. How are files and chunks are deleted? What s the advantages of the delayed space reclamation (garbage collection), rather than eager deletion? File: When a file is deleted by the application, the master logs the deletion immediately. The file is just renamed to a hidden name that includes the deletion timestamp. During the master s regular scan of the file system namespace, it removes any such hidden files if they have existed for more than three days. Then remove namespace, metadata, etc. Chunk: the master identifies not reachable chunks with heartbeat message and erases the metadata for those chunks.

47 Question [12] After a file is deleted, GFS does not immediately reclaim the available physical storage. It does so only lazily during regular garbage collection at both the file and chunk levels. How are files and chunks are deleted? What s the advantages of the delayed space reclamation (garbage collection), rather than eager deletion? Advantages: 1, simple and reliable for large distribute systems 2, it merges storage reclamation into the regular background activities of the master, less overhead or burden for master node 3, avoid accidental, irreversible deletion

48 Outline GFS Background, Concepts and Key words Example of GFS Operations Some optimizations in GFS Evaluation Conclusion

49 Evaluation Environment Cluster - 1 master - 16 chunkservers (1.4GHz PIII CPU, 2G Ram, 2*80GB Disk, 100Mpbs Ethernet) - 16 clients Server machines connected to central switch by 100 Mbps Ethernet Switches (HP2524) connected with 1 Gbps link

50 Aggregate Throughputs N clients reading 4 MB region from 320 GB file set simultaneously. Read rate slightly lower as clients go up due to probability reading from same chunkserver 1 client: - 10MB/s, 80% limit 16 client: - 6MB/s, 75% limit

51 Aggregate Throughputs N clients writing to N files simultaneously. Low write rate is due to delay in propagating data among replicas. Slow write is not major problem with aggregate write bandwidth to large clients. 1 client: MB/s, 50% limit 16 client: MB/s per client

52 Aggregate Throughputs N clients appending to a single file simultaneously. Append rate slightly lower as clients go up due to network congestion by different clients. Chunkserver network congestion is not major issue with large clients appending to large shared files. 1 client: - 6 MB/s 16 client: MB/s per client

53 Real World Clusters A: research and development B: production data processing

54 GFS Deployment in Google Many GFS clusters Hundreds/thousands of storage nodes each Managing petabytes of data GFS is under BigTable, etc.

55 Outline GFS Background, Concepts and Key words Example of GFS Operations Some optimizations in GFS Evaluation Conclusion

56 Conclusion Google File System is a scalable distributed file system for large distributed data-intensive applications, which runs on inexpensive commodity hardware and provides fault tolerance, high performance to a large number of clients. GFS shares many of the same goals as previous distributed file systems but has its own innovations and limitations (master bottleneck, designed for large files, hotspot, etc) GFS meets Google s storage needs and serves Google s apps and services

57 One Comparison Taobao File System from Alibaba Hundreds of Millions of Products Product images, description, comments, transactions, etc. are all small files.

58 Taobao File System Optimization for small files Open sourced 1st level index One chunk contains many small files with hierarchy Nth level index

59 Reference cs.brown.edu/~debrabant/cis570-website/slides/ gfs.ppt cmsc818k/lectures/gfs-hdfs.pdf google-file-system-gfs-presentation

60 Q & A

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