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1、GFS: The Google File System,Brad Karp UCL Computer Science,CS Z03 / 4030 30th October, 2006,2,Motivating Application: Google,Crawl the whole web Store it all on “one big disk” Process users searches on “one big CPU” More storage, CPU required than one PC can offer Custom parallel supercomputer: expe

2、nsive (so much so not really available today),3,Cluster of PCs as Supercomputer,Lots of cheap PCs, each with disk and CPU High aggregate storage capacity Spread search processing across many CPUs How to share data among PCs? Ivy: shared virtual memory Fine-grained, relatively strong consistency at l

3、oad/store level Fault tolerance? NFS: share fs from one server, many clients Goal: mimic original UNIX local fs semantics Compromise: close-to-open consistency (performance) Fault tolerance?,4,Google Platform Characteristics,100s to 1000s of PCs in cluster Cheap, commodity parts in PCs Many modes of

4、 failure for each PC: App bugs, OS bugs Human error Disk failure, memory failure, net failure, power supply failure Connector failure Monitoring, fault tolerance, auto-recovery essential,5,Google File System: Design Criteria,Detect, tolerate, recover from failures automatically Large files, = 100 MB

5、 in size Large, streaming reads (= 1 MB in size) Read once Large, sequential writes that append Write once Concurrent appends by multiple clients (e.g., producer-consumer queues) Want atomicity for appends without synchronization overhead among clients,6,GFS: Architecture,One master server (state re

6、plicated on backups) Many chunk servers (100s 1000s) Spread across racks; intra-rack b/w greater than inter-rack Chunk: 64 MB portion of file, identified by 64-bit, globally unique ID Many clients accessing same and different files stored on same cluster,7,GFS: Architecture (2),8,Master Server,Holds

7、 all metadata: Namespace (directory hierarchy) Access control information (per-file) Mapping from files to chunks Current locations of chunks (chunkservers) Manages chunk leases to chunkservers Garbage collects orphaned chunks Migrates chunks between chunkservers,9,Chunkserver,Stores 64 MB file chun

8、ks on local disk using standard Linux filesystem, each with version number and checksum Read/write requests specify chunk handle and byte range Chunks replicated on configurable number of chunkservers (default: 3) No caching of file data (beyond standard Linux buffer cache),10,Client,Issues control

9、(metadata) requests to master server Issues data requests directly to chunkservers Caches metadata Does no caching of data No consistency difficulties among clients Streaming reads (read once) and append writes (write once) dont benefit much from caching at client,11,Client API,Is GFS a filesystem i

10、n traditional sense? Implemented in kernel, under vnode layer? Mimics UNIX semantics? No; a library apps can link in for storage access API: open, delete, read, write (as expected) snapshot: quickly create copy of file append: at least once, possibly with gaps and/or inconsistencies among clients,12

11、,Client Read,Client sends master: read(file name, chunk index) Masters reply: chunk ID, chunk version number, locations of replicas Client sends “closest” chunkserver w/replica: read(chunk ID, byte range) “Closest” determined by IP address on simple rack-based network topology Chunkserver replies wi

12、th data,13,Client Write,Some chunkserver is primary for each chunk Master grants lease to primary (typically for 60 sec.) Leases renewed using periodic heartbeat messages between master and chunkservers Client asks server for primary and secondary replicas for each chunk Client sends data to replica

13、s in daisy chain Pipelined: each replica forwards as it receives Takes advantage of full-duplex Ethernet links,14,Client Write (2),All replicas acknowledge data write to client Client sends write request to primary Primary assigns serial number to write request, providing ordering Primary forwards w

14、rite request with same serial number to secondaries Secondaries all reply to primary after completing write Primary replies to client,15,Client Write (3),16,GFS: Consistency Model,Changes to namespace (i.e., metadata) are atomic Done by single master server! Master uses log to define global total or

15、der of namespace-changing operations Data changes more complicated Consistent: file region all clients see as same, regardless of replicas they read from Defined: after data mutation, file region that is consistent, and all clients see that entire mutation,17,GFS: Data Mutation Consistency,Record ap

16、pend completes at least once, at offset of GFS choosing Apps must cope with Record Append semantics,18,Applications andRecord Append Semantics,Applications should include checksums in records they write using Record Append Reader can identify padding / record fragments using checksums If application

17、 cannot tolerate duplicated records, should include unique ID in record Reader can use unique IDs to filter duplicates,19,Logging at Master,Master has all metadata information Lose it, and youve lost the filesystem! Master logs all client requests to disk sequentially Replicates log entries to remot

18、e backup servers Only replies to client after log entries safe on disk on self and backups!,20,Chunk Leases and Version Numbers,If no outstanding lease when client requests write, master grants new one Chunks have version numbers Stored on disk at master and chunkservers Each time master grants new

19、lease, increments version, informs all replicas Master can revoke leases e.g., when client requests rename or snapshot of file,21,What If the Master Reboots?,Replays log from disk Recovers namespace (directory) information Recovers file-to-chunk-ID mapping Asks chunkservers which chunks they hold Re

20、covers chunk-ID-to-chunkserver mapping If chunk server has older chunk, its stale Chunk server down at lease renewal If chunk server has newer chunk, adopt its version number Master may have failed while granting lease,22,What if Chunkserver Fails?,Master notices missing heartbeats Master decrements

21、 count of replicas for all chunks on dead chunkserver Master re-replicates chunks missing replicas in background Highest priority for chunks missing greatest number of replicas,23,File Deletion,When client deletes file: Master records deletion in its log File renamed to hidden name including deletio

22、n timestamp Master scans file namespace in background: Removes files with such names if deleted for longer than 3 days (configurable) In-memory metadata erased Master scans chunk namespace in background: Removes unreferenced chunks from chunkservers,24,GFS: Summary,Success: used actively by Google t

23、o support search service and other applications Availability and recoverability on cheap hardware High throughput by decoupling control and data Supports massive data sets and concurrent appends Semantics not transparent to apps Must verify file contents to avoid inconsistent regions, repeated appends (at-least-once semantics) Performance not good for all apps Assumes read-once, write-once workload (no client caching!),