Outline
• Announcements
• Lab2
• Distributed File Systems
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Announcements
• The midterm will be on March 20
– It will cover up Chap 8
• The emphasis will be on the materials covered in class
• You are not required to know the details of distributed deadlock
detection algorithms
– There will be a review on March 18
• Regarding the first Lab 1
– Each group needs to set up an appointment with me to do a
demonstration of your lab for grading
– I want to remind you that I take cheating very seriously
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Distributed File Systems
• A distributed file system is a resource
management component in a distributed
operating systems
– It implements a common file system shared by
all the computers in the systems
• Two important goals
– Network transparency
– High availability
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Architecture
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Architecture – cont.
• Normally for performance reasons distributed
file systems are organized as a client-server
architecture
– File servers store files and perform storage and
retrieval upon client’s requests
– Two most important parts are
• Name server
• Cache manager
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Architecture – cont.
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Architecture – cont.
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Mounting
• Mounting is a way to bind together different
file systems to form a single hierarchical
structured name space
– It is widely used in both local and distributed
UNIX machines
– In distributed file systems, file systems
maintained by remote servers are mounted at the
clients
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Mounting – cont.
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Mounting – cont.
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Mounting – cont.
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Mounting – cont.
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Automounting
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Automounting – cont.
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Caching
• Caching is commonly used in distributed file
systems to reduce delays in accessing the
data
– In file caching, a copy of the data stored at a
remote file server is brought to the client,
reducing access delays due to network latency
– The effectiveness of caching is based on the
temporal locality in programs
– Files can also be cached at the server side
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Client Caching
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Client Caching – cont.
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Cache Consistency
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Hints
• An alternative approach to caching
– The cached data is treated as hints
– The cached data is not guaranteed to be
completely accurate
• The cache consistency issue is ignored in this
implementation
– This is useful for applications which can recover
from invalid cached data
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Bulk Data Transfer
• Bulk data transfer is to transfer multiple data
blocks instead of just the block being
referenced by the client
– Temporal locality and the fact that most files are
accessed in their entirety
– Reduce the network communication overhead by
reducing the cost of executing communication
protocols
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Security
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Naming in Distributed File Systems
• A name in file systems is a way to reference a
file or a directory
• Name resolution refers to the process of
mapping a name to an object (or in the case
of replication, to multiple objects)
• A name space is a collection of names
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Naming in a Local File System
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Naming in a Local File System – cont.
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Naming in Distributed File Systems – cont.
• Three approaches to naming in distributed
file systems
– The simplest scheme is to concatenate the host
name to the names of files
• Not network transparent
• Not location-independent
– Mounting remote directories to local directories
• Location transparent but not network transparent
– A single global directory
• Limited to a few cooperating computers
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Naming in Distributed File Systems – cont.
• Context
– Content can be used to partition a file name space
– Here a filename consists of a context and a name
local to the context
– Name resolution involves interpreting the name
within a context, which may invoke other contexts
recursively
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Naming in Distributed File Systems – cont.
• Name Servers are responsible for name
resolution in distributed file systems
– A name server is a process that maps names
specified by clients to stored objects such as files
and directories
– A single name server vs. multiple name servers
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Caches on Disk or Memory
• Cache in main memory vs. cache on a local
disk
– Cache in main memory
• Advantages
• Disadvantages
– Cache on a local disk
• Advantages
• Disadvantages
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Writing Policy
• This is related to the cache consistency
– It decides what to do when a cache block at the
client is modified
– Several different policies
• Write-through
• Delayed writing policy for some time
– Delayed writing policy when the file is closed
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Cache Consistency
• Schemes to guarantee consistency
– Server-initiated approach
• Servers inform the cache managers whenever the data
in client caches become stale
• Cache managers can retrieve the new data when
needed
– Client-initiated approach
• Cache managers validate data with the server before
returning it to the clients
– Limited caching
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Availability
• Availability is an important issue in
distributed file systems
– Replication is the primary mechanism for
enhancing the availability of files in distributed
file systems
• Replication
– Unit of replication
– Replica management
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Scalability
• Scalability deals with the suitability of the
design to support more clients
–
–
–
–
Caching helps reduce the client response time
Server-initiated cache invalidation
Some clients can be used as servers
The structure of the server process also plays a
major role in scalability
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Semantics
• Semantics of a file system characterize the
effects of accesses on files
– For example, a read operation should return the
data (stored) due to the latest write operation
– Guaranteeing the semantics when employing
caching, is difficult and expensive
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