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Lecture Operating systems: A concept-based approach (2/e): Chapter 19 - Dhananjay M. Dhamdhere

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Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 1
Copyright © 2008


Design issues in Distributed File Systems



Transparency of a file system
– A user need not know the location of a file in the system
* Location transparency: The name of a file should not reveal its

location
 Provides user convenience
* Location independence: File system should be able to change the
location of a file without having to change its name
 Enables the file system to optimize its own performance

Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 2
Copyright © 2008


Design issues in Distributed File Systems



Fault tolerance
– Two techniques are used to ensure that a fault does not disrupt
operation of a file system
* Journaling technique may be used to ensure consistency of meta
data
* Stateless file server design eliminates the need to maintain
consistency of meta data

Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 3
Copyright © 2008


Design issues in Distributed File Systems




Performance
– File system performance has two aspects
* High efficiency
 File caching boosts efficiency by reducing network traffic
* Scalability
 Response time should not degrade as system size grows
 Special scalability techniques are employed
» Clusters of computers
» Distributed locking techniques

Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 4
Copyright © 2008


Basics of file processing in a DFS






A user or process that accesses a file is called a client
When the client opens a file, the DFS finds its location during name resolution
DFS sets up the arrangement involving the client and file server agents
This arrangement is analogous to a remote procedure call (RPC)

Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 5
Copyright © 2008


Transparency



DFS may use the following arrangement
– Each file is assigned a globally unique file id
* It is stored in the directory entry of the file

– DFS uses a separate data structure to hold file id-location pairs
* If the DFS changes a file’s location, it updates its entry in this data
structure

Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 6
Copyright © 2008


File sharing semantics



Specify how file updates are visible to concurrent users
– Unix semantics
* Use single image mutable files
 Updates made by one client are immediately visible to others

– Session semantics
* Employ multiple image mutable files
 Clients in a session use a single image mutable file
» Their updates are visible to one another immediately but
are visible to other clients only when the session ends

– Transaction semantics
* File processing performed by a client is implemented as an atomic
transaction, so only one client can access a file at any time
* Provides reliability when faults occur
Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 7
Copyright © 2008


Session semantics

•  Only clients in the same node can form a session
• A new session is started in a node if no session is in progress in it, or if some
client of a previous session has updated the file and performed a close operation
• The semantics do not guarantee portability because they do not specify which
file version should be accessed by a new client

Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 8
Copyright © 2008


Fault tolerance



File system reliability has many facets. A file should be
– Robust
* A file must survive faults in a guaranteed manner
 Techniques such as disk mirroring ensure robustness

– Recoverable
* It should be possible to restore it to an earlier state when a fault
occurs

– Available
* It should be accessible despite faults
 Availability is typically ensured while opening a file
 Ensuring it during file processing would require a complex
arrangement (see quorum algorithms in Chapter 18)
Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 9
Copyright © 2008


Fault tolerance techniques



Three key techniques employed for fault tolerance
– Cached directories
* A remote directory is cached during name resolution
 helps in tolerating failures of intermediate nodes in future name
resolutions

– File replication
* Quorum algorithms can be used to control read / write accesses;
however, they are expensive
 Instead, only the primary copy of a file may be updated and the
updates are propagated to other copies

– Stateless file server
* Does not maintain any meta data
 Hence immune to consistency problems when faults arise
Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 10
Copyright © 2008


Stateless file servers



A conventional file system performs file processing in a
stateful manner
– It maintains information about the activity in memory
* For example, file control block, file map table, file buffers



– Loss of this information disrupts file processing activities

Stateless file server
– Does not maintain any information about the activity in memory
* Extra work may be performed to obtain relevant information such as
FMT at every file operation
 To avoid it, file server may return some information to the client
process, which can provide it at the next operation
 Hint: Server maintains some of the information and uses it if
available; otherwise, it acts as a stateless server

Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 11
Copyright © 2008


Performance techniques of distributed file systems



Three classes of techniques
– Efficiency of file access
* Multi-threaded file server
 Threads can service file access requests concurrently
* Hint-based file server
 Hybrid of stateful and stateless file server
 Uses a hint if available, else functions as a stateless server

– File caching
* Reduces network traffic by caching portions of a file (see next slide)

– Scalability
* Clusters of nodes using high speed LANs are used
 Processes in a cluster rarely need to access outside files
 Hence network traffic does not grow with number of clusters
Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 12
Copyright © 2008


A schematic of file caching






Cache manager in client node traps file accesses whose data exists in the cache
Cache validation traffic is needed to ensure validity of cached data
Client-initiated validation is performed by the cache manager in client site
In server-initiated validation, server keeps track of copies of data in the caches
and invalidates the copies when data is updated

Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 13
Copyright © 2008


Sun Network File System



Provides a stateless file server for files on a network
– Virtual file system (VFS) layer
* It uses the mount protocol and creates system-wide unique file ids
* Export list of a node identifies a local directory and which remote
nodes can access it
* Permits cascaded mounting of file systems, i.e., mounting over a
mounted file system

– Network file system (NFS) layer
* Uses a directory names cache to perform path name resolution
* Uses the NFS protocol to provide access to a remote file using RPC
 NFS employs file caching to provide high performance

Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 14
Copyright © 2008


Architecture of the Sun network file system (NFS)

• The VFS interface either invokes a local file system or the NFS
• NFS uses the RPC protocol to implement file operations
• VFS interface acts as both a client and a server
Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 15
Copyright © 2008


Andrew and Coda file systems



Features
– Scalable performance is obtained through use of clusters and
caching of whole files on nodes within a cluster
– Files of a user exist in one volume
* Volumes can be mounted and migrated within the system

– File sharing semantics
* File is cached in units called chunks; chunk size is adapted
* Server-initiated cache validation using callbacks
 Node caching a file F has a callback on F
 Callback is broken when F is updated

– Features of Coda
* Provides replication using read-one-write-all policy
* Supports disconnected mode of operation
Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 16
Copyright © 2008


General parallel file system (GPFS)



File system for clusters operating under Linux
– Uses data striping; uses large block size to reduce seek
overhead and small block size for high transfer rate for small
files
– Uses locking to maintain consistency of file data
* Lock granularity is adjusted to trade-off overhead and concurrency
 Implemented using lock token with an adjustable byte-range

– Uses partitioned free disk space map to provide concurrency
– Each node writes a journal for recovery
– A failure may partition the cluster
* Only majority partition can continue file processing activities
 Prevents file inconsistency due to concurrent updates
Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 17
Copyright © 2008


Windows



Features for replication and distribution
– Remote differential compression (RDC)
* Reduces the replication and file coherence traffic between servers
 Replication is performed using the notion of replication group
 RDC protocol synchronizes copies of a replicated folder by
transmitting changes in it

– DFS namespaces enables list of virtual tree of folders that can
be accessed by a client located in any node
* System administrator associates a list of servers with a folder
* This list is given to the client that wishes to access the folder
 The client contacts the servers in this order to access the folder
 This scheme permits a hot standby arrangement
Chapter 19: 
Distributed File Systems

Dhamdhere: Operating Systems—
A Concept­Based Approach, 2 ed

Slide No: 18
Copyright © 2008



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