1. Outline
Review of Quiz #1
Distributed File Systems
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2. 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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3. Architecture
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4. 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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5. Architecture – cont.
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6. Architecture – cont.
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7. 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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8. Mounting – cont.
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9. Mounting – cont.
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10. Mounting – cont.
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11. Mounting – cont.
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12. Automounting - cont.
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13. Automounting – cont.
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14. 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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15. Client Caching
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16. Client Caching – cont.
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17. Cache Consistency
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18. 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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19. 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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20. Security
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21. 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 collection of names
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22. Naming in a Local File System
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23. Naming in a Local File System – cont.
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24. 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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25. 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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26. 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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27. 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
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28. 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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29. 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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30. 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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31. 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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32. 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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