Outline Distributed File Systems continued 01 16 19 COP5611 1 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 01 16 19 COP5611 2 Architecture 01 16 19 COP5611 3 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 01 16 19 COP5611 4 Architecture cont 01 16 19 COP5611 5 Architecture cont 01 16 19 COP5611 6 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 01 16 19 COP5611 7 Mounting cont 01 16 19 COP5611 8 Mounting cont 01 16 19 COP5611 9 Mounting cont 01 16 19 COP5611 10 Mounting cont 01 16 19 COP5611 11 Automounting 01 16 19 COP5611 cont 12 Automounting cont 01 16 19 COP5611 13 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 01 16 19 COP5611 14 Client Caching 01 16 19 COP5611 15 Client Caching cont 01 16 19 COP5611 16 Cache Consistency 01 16 19 COP5611 17 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 01 16 19 COP5611 18 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 01 16 19 COP5611 19 Security 01 16 19 COP5611 20 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 01 16 19 COP5611 21 Naming in a Local File System 01 16 19 COP5611 22 Naming in a Local File System cont 01 16 19 COP5611 23 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 01 16 19 COP5611 24 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 01 16 19 COP5611 25 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 01 16 19 COP5611 26 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 01 16 19 COP5611 27 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 01 16 19 COP5611 28 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 01 16 19 COP5611 29 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 01 16 19 COP5611 30 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 01 16 19 COP5611 31 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 01 16 19 COP5611 32 Case Studies There are distributed file systems that have been developed Architecture Communication Processes and their organization Naming Consistency Caching and replication Fault tolerance Security 01 16 19 COP5611 33 Case Studies cont These examples are taken from Tanenbaum and van Steen s book It does not follow the main textbook because The material there is not up to date It did not provide enough details You need to read Sect 9 5 01 16 19 COP5611 34 Sun Network File Systems Developed by Sun The first version was developed and was kept to Sun The second version was incorporated in SunOS 2 0 Version 3 was released around 1994 Version 4 is under development to make the NFS a true wide area file system across the Internet 01 16 19 COP5611 35 Sun Network File Systems cont NFS is not a true file system It is a collection of protocols that together provide clients with a model of a distributed files system In some sense it is a middle ware like CORBA NFS protocols are designed in such a way that different implementations should easily interoperate It can run on a heterogeneous collection of computers
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