Distributed File System.ppt
- 1. Distributed File Systems (DFS) A RESOURCE MANAGEMENT COMPONENT OF A DISTRIBUTED OPERATING SYSTEM
- 2. Achievements through DFS Two important goals of distributed file systems Network Transparency To provide the same functional capabilities to access files distributed over a network Users do not have to be aware of the location of files to access them High Availability Users should have the same easy access to files, irrespective of their physical location System failures or regularly scheduled activities such as backups or maintenance should not result in the unavailability of files
- 3. Architecture Files can be stored at any machine and computation can be performed at any machine A machine can access a file stored on a remote machine where the file access operations are performed and the data is returned Alternatively, File Servers are provided as dedicated to storing files and performing storage and retrieval operations Two most important services in a DFS are Name Server: a process that maps names specified by clients to stored objects, e.g. files and directories Cache Manager: a process that implements file caching, i.e. copying a remote file to the client’s machine when referred by the client
- 5. Data Access Actions in DFS
- 6. Mechanisms for Building DFS Mounting Allows the binding together of different filename spaces to form a single hierarchically structured name space Kernel maintains a structure called the mount table which maps mount points to appropriate storage devices. Caching To reduce delays in the accessing of data by exploiting the temporal locality of reference exhibited by program Hints An alternative to cached data to overcome inconsistency problem when multiple clients access shared data Bulk Data Transfer To overcome the high cost of executing communication protocols, i.e. assembly/disassembly of packets, copying of buffers between layers Encryption To enforce security in distributed systems with a scenario that two entities wishing to communicate establish a key for conversation
- 7. Design Goals Naming and Name Resolution Caches on Disk or Main Memory Writing Policy Cache Consistency Availability Scalability Semantics
- 8. Naming and Name Resolution Name in file systems is associated with an object (e.g. a file or a directory) Name resolution refers to the process of mapping a name to an object, or in case of replication, to multiple objects. Name space is a collection of names which may or may not share an identical resolution mechanism Three approaches to name files in DE Concatenation Mounting (Sun NFS) Directory structured (Sprite and Apollo) The Concepts of Contexts A context identifies the name space in which to resolve a given name Examples: x-Kernel Logical File System, Tilde Naming Scheme Name Server Resolves the names in distributed systems. Drawbacks involved such as single point of failure, performance bottleneck. Alternate is to have several name servers, e.g. Domain Name Servers
- 9. Caches on Disk or Main Memory Cache in Main Memory Diskless workstations can also take advantage of caching Accessing a cache is much faster than access a cache on local disk The server-cache is in the main memory, and hence a single cache design for both Disadvantages It competes with the virtual memory system for physical memory space A more complex cache manager and memory management system Large files cannot be cached completely in memory Cache in Local Disk Large files can be cached without affecting performance Virtual memory management is simple Example: Coda File System
- 10. Writing Policy Decision to when the modified cache block at a client should be transferred to the server Write-through policy All writes requested by the applications at clients are also carried out at the server immediately. Delayed writing policy Modifications due to a write are reflected at the server after some delay. Write on close policy The updating of the files at the server is not done until the file is closed
- 11. Cache Consistency Two approaches to guarantee that the data returned to the client is valid. Server-initiated approach Server inform cache managers whenever the data in the client caches become stale Cache managers at clients can then retrieve the new data or invalidate the blocks containing the old data Client-initiated approach The responsibility of the cache managers at the clients to validate data with the server before returning it Both are expensive since communication cost is high Concurrent-write sharing approach A file is open at multiple clients and at least one has it open for writing. When this occurs for a file, the file server informs all the clients to purge their cached data items belonging to that file. Sequential-write sharing issues causing cache inconsistency Client opens a file, it may have outdated blocks in its cache Client opens a file, the current data block may still be in another client’s cache waiting to be flushed. (e.g. happens in Delayed writing policy)
- 12. Availability Immunity to the failure of server of the communication network Replication is used for enhancing the availability of files at different servers It is expensive because Extra storage space required The overhead incurred in maintaining all the replicas up to date Issues involve How to keep the replicas of a file consistent How to detect inconsistencies among replicas of a file and recover from these inconsistencies Causes of Inconsistency A replica is not updated due to failure of server All the file servers are not reachable from all the clients due to network partition The replicas of a file in different partitions are updated differently
- 13. Availability (contd.) Unit of Replication The most basic unit is a file A group of files of a single user or the files that are in a server (the group file is referred to as volume, e.g. Coda) Combination of two techniques, as in Locus Replica Management The maintenance of replicas and in making use of them to provide increased availability Concerns with the consistency among replicas A weighted voting scheme (e.g. Roe File System) Designated agents scheme (e.g. Locus) Backups servers scheme (e.g. Harp File System)
- 14. Scalability The suitability of the design of a system to cater to the demands of a growing system As the system grow larger, both the size of the server state and the load due to invalidations increase The structure of the server process also plays a major role in deciding how many clients a server can support If the server is designed with a single process, then many clients have to wait for a long time whenever a disk I/O is initiated These waits can be avoided if a separate process is assigned to each client A significant overhead due to the frequent context switches to handle requests from different clients can slow down the server An alternate is to use Lightweight processes (threads)
- 15. Semantics The semantics of a file system characterizes the effects of accesses on files Guaranteeing the semantics in distributed file systems, which employ caching, is difficult and expensive In server-initiated cache the invalidation may not occur immediately after updates and before reads occur at clients. This is due to communication delays To guarantee the above semantics all the reads and writes from various clients will have to go through the server Or sharing will have to be disallowed either by the server, or by the use of locks by applications
- 16. Student’s Task CASE STUDIES 9.5.1 THE SUN NETWORK FILE SYSTEM