3. Operating systems
• Operating system - Software that supervises
and controls tasks on a computer. Individual
OS:
– Batch processing jobs are collected, placed in
a queue, no interaction with job during processing
– Time shared computing resources are provided
to different users, interaction with program during
execution
– RT systems fast response, can be interrupted
4. Distributed Systems
• Consists of a number of computers that are connected and
managed so that they automatically share the job processing
load among the constituent computers.
• A distributed operating system is one that appears to its users as
a traditional uniprocessor system, even though it is actually
composed of multiple processors.
• It gives a single system view to its users and provides a single
service.
• Users are transparent to location of files. It provides a virtual
computing env.
Eg The Internet, ATM banking networks, mobile computing
networks, Global Positioning Systems and Air Traffic Control
DISTRIBUTED SYSTEM IS A COLLECTION OF INDEPENDENT
COMPUTERS THAT APPEARS TO IS USERS AS A SINGLE
COHERENT SYSTEM
5. Network Operating System
• In a network operating system the users are aware
of the existence of multiple computers.
• The operating system of individual computers must
have facilities to have communication and
functionality.
• Each machine runs its own OS and has its own user.
• Remote login and file access
• Less transparent but more independency
Applicatio
n
Applicatio
n
Applicatio
n
Distributed Operating System Services
Application Application Application
Network
OS
Network
OS
Network
OS
Distributed OS Networked OS
6. DFS
• Resource sharing is the motivation behind distributed
Systems. To share files file system
• File System is responsible for the organization, storage,
retrieval, naming, sharing, and protection of files.
• The file system is responsible for controlling access to
the data and for performing low-level operations such as
buffering frequently used data and issuing disk I/O
requests
• The goal is to allow users of physically distributed
computers to share data and storage resources by
using a common file system.
7. Hadoop
What is Hadoop?
It's a framework for running applications on large clusters of
commodity hardware which produces huge data and to
process it
Apache Software Foundation Project
Open source
Amazon’s EC2
alpha (0.18) release available for download
Hadoop Includes
HDFS a distributed filesystem
Map/Reduce HDFS implements this programming model. It
is an offline computing engine
Concept
Moving computation is more efficient than moving large
data
8. • Data intensive applications with Petabytes of data.
• Web pages - 20+ billion web pages x 20KB = 400+
terabytes
– One computer can read 30-35 MB/sec from disk
~four months to read the web
– same problem with 1000 machines, < 3 hours
• Difficulty with a large number of machines
– communication and coordination
– recovering from machine failure
– status reporting
– debugging
– optimization
– locality
9. FACTS
Single-thread performance doesn’t matter
We have large problems and total throughput/price more
important than peak performance
Stuff Breaks – more reliability
• If you have one server, it may stay up three years (1,000 days)
• If you have 10,000 servers, expect to lose ten a day
“Ultra-reliable” hardware doesn’t really help
At large scales, super-fancy reliable hardware still fails, albeit
less often
– software still needs to be fault-tolerant
– commodity machines without fancy hardware give better
perf/price
DECISION : COMMODITY HARDWARE.
DFS : HADOOP – REASONS?????
10. HDFS Why? Seek vs Transfer
• CPU & transfer speed, RAM & disk size double every 18
- 24 months
• Seek time nearly constant (~5%/year)
• Time to read entire drive is growing vs transfer rate.
• Moral: scalable computing must go at transfer rate
• BTree (Relational DBS)
– operate at seek rate, log(N) seeks/access
-- memory / stream based
• sort/merge flat files (MapReduce)
– operate at transfer rate, log(N) transfers/sort
-- Batch based
12. Characteristics
• Fault tolerant, scalable, Efficient, reliable distributed
storage system
• Moving computation to place of data
• Single cluster with computation and data.
• Process huge amounts of data.
• Scalable: store and process petabytes of data.
• Economical:
– It distributes the data and processing across clusters of
commonly available computers.
– Clusters PCs into a storage and computing platform.
– It minimises no of CPU cycles, RAM on individual
machines etc.
• Efficient:
– By distributing the data, Hadoop can process it in parallel on
the nodes where the data is located. This makes it extremely
rapid.
– Computation is moved to place where data is present.
• Reliable:
– Hadoop automatically maintains multiple copies of data
– Automatically redeploys computing tasks based on failures.
14. • Data Model
– Data is organized into files and directories
– Files are divided into uniform sized blocks and
distributed across cluster nodes
– Replicate blocks to handle hardware failure
– Checksums of data for corruption detection
and recovery
– Expose block placement so that computes
can be migrated to data
• large streaming reads and small random reads
• Facility for multiple clients to append to a file
15. • Assumes commodity hardware that fails
– Files are replicated to handle hardware
failure
– Checksums for corruption detection and
recovery
– Continues operation as nodes / racks added
/ removed
• Optimized for fast batch processing
– Data location exposed to allow computes to
move to data
– Stores data in chunks/blocks on every node
in the cluster
– Provides VERY high aggregate bandwidth
16. • Files are broken in to large blocks.
– Typically 128 MB block size
– Blocks are replicated for reliability
– One replica on local node,
another replica on a remote rack,
Third replica on local rack,
Additional replicas are randomly placed
• Understands rack locality
– Data placement exposed so that computation can be
migrated to data
• Client talks to both NameNode and DataNodes
– Data is not sent through the namenode, clients
access data directly from DataNode
– Throughput of file system scales nearly linearly with
the number of nodes.
19. Components
• DFS Master “Namenode”
– Manages the file system namespace
– Controls read/write access to files
– Manages block replication
– Checkpoints namespace and journals
namespace changes for reliability
Metadata of Name node in Memory
– The entire metadata is in main memory
– No demand paging of FS metadata
Types of Metadata:
List of files, file and chunk namespaces; list of
blocks, location of replicas; file attributes etc.
20. DFS SLAVES or DATA NODES
• Serve read/write requests from clients
• Perform replication tasks upon instruction by
namenode
Data nodes act as:
1) A Block Server
– Stores data in the local file system
– Stores metadata of a block (e.g. CRC)
– Serves data and metadata to Clients
2) Block Report: Periodically sends a report of all
existing blocks to the NameNode
3) Periodically sends heartbeat to NameNode (detect
node failures)
4) Facilitates Pipelining of Data (to other specified
DataNodes)
21. • Map/Reduce Master “Jobtracker”
– Accepts MR jobs submitted by users
– Assigns Map and Reduce tasks to Tasktrackers
– Monitors task and tasktracker status, re-
executes tasks upon failure
• Map/Reduce Slaves “Tasktrackers”
– Run Map and Reduce tasks upon instruction
from the Jobtracker
– Manage storage and transmission of
intermediate output.
22. SECONDARY NAME NODE
• Copies FsImage and Transaction Log from
NameNode to a temporary directory
• Merges FSImage and Transaction Log into
a new FSImage in temporary directory
• Uploads new FSImage to the NameNode
– Transaction Log on NameNode is purged
23. HDFS Architecture
• NameNode: filename, offset> blockid, block > datanode
• DataNode: maps block > local disk
• Secondary NameNode: periodically merges edit logs
Block is also called chunk
25. HDFS API
• Most common file and directory operations
supported:
– Create, open, close, read, write, seek, list,
delete etc.
• Files are write once and have exclusively
one writer
• Some operations peculiar to HDFS:
– set replication, get block locations
• Support for owners, permissions
26. DATA CORRECTNESS
• Use Checksums to validate data
– Use CRC32
• File Creation
– Client computes checksum per 512 byte
– DataNode stores the checksum
• File access
– Client retrieves the data and checksum from
DataNode
– If Validation fails, Client tries other replicas
27. MUTATION ORDER AND LEASES
• A mutation is an operation that changes the
contents / metadata of a chunk such as append /
write operation.
• Each mutation is performed at all replicas.
• Leases (order of mutations) are used to maintain
consistency
• Master grants chunk lease to one replica
(primary)
• Primary picks the serial order for all mutations to
the chunk
• All replicas follow this order (consistency)
28.
29. Software Model - ???
• Parallel programming improves performance and
efficiency.
• In a parallel program, the processing is broken up into
parts, each of which can be executed concurrently
• Identify whether the problem can be parallelised (fib)
• Matrix operations with independency
30. Master/Worker
• The MASTER:
– initializes the array and splits it up according
to the number of available WORKERS
– sends each WORKER its subarray
– receives the results from each WORKER
• The WORKER:
– receives the subarray from the MASTER
– performs processing on the subarray
– returns results to MASTER
31. The area of the square, denoted
As = (2r)^2 or 4r^2.
The area of the circle, denoted
Ac, is pi * r2.
• pi = Ac / r^2
• As = 4r^2
• r^2 = As / 4
• pi = 4 * Ac / As
• pi= 4 * No of pts on
the circle / num of
points on the square
CALCULATING PI
32. • Randomly generate points in the square
• Count the number of generated points that are
both in the circle and in the square MAP
(find ra = No of pts on the circle / num of points
on the square)
• ra = the number of points in the circle divided
by the number of points in the square
gather all ra
• PI = 4 * r REDUCE
Parallelised calculation of points on the circle
(MAP)
Then merged in to find PI REDUCE
34. WHAT IS MAP REDUCE PROGRAMMING
• Restricted parallel programming model meant
for large clusters
– User implements Map() and Reduce()
• Parallel computing framework (HDFS lib)
– Libraries take care of EVERYTHING else
(abstraction)
• Parallelization
• Fault Tolerance
• Data Distribution
• Load Balancing
• Useful model for many practical tasks
