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5.distributed file systems
Distributed file systems allow files to be shared across multiple computers even without other inter-process communication. There are three main naming schemes for distributed files: 1) mounting remote directories locally, 2) combining host name and local name, and 3) using a single global namespace. File caching schemes aim to reduce network traffic by storing recently accessed files in local memory. Key decisions for caching schemes include cache location (client/server memory or disk) and how/when modifications are propagated to servers.
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5.distributed file systems
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- 2. Distributed Files Systems(DFS) • Allows multi-computer systems to share files – Even when no other IPC or RPC is needed • Sharing devices – Special case of sharing files • E.g., – NFS (Sun’s Network File System) – Windows NT, 2000, XP – Andrew File System (AFS) & others …
- 3. Naming of DistributedFiles • Naming – mapping between logical and physical objects • A transparent DFS hides the location where in the network the file is stored. • Location transparency – file name does not reveal the file’s physical storage location. • Location independence – file name does not need to be changed when the file’s physical storage location changes. – Better file abstraction. – Separates the naming hierarchy from the storage-devices hierarchy
- 4. DFS – ThreeNaming Schemes 1. Mount remote directories to local directories, giving the appearance of a coherent local directory tree • Mounted remote directories can be accessed transparently. • Unix/Linux with NFS; Windows with mapped drives 2. Files named by combination of host name and local name; • Guarantees a unique system wide name • Windows Network Places, Apollo Domain 3. Total integration of component file systems. • A single global name structure spans all the files in the system. 9
- 5. Mounting Remote Directories(NFS) .
- 6. File caching schemes Contents Introduction Key decisions of file caching schemes Cache location Modification propagation
- 7. Introduction To Retain recentlyaccessed files in the main memory. Repeated access to the same information can be handled locally. Reduce network traffic.
- 8. Cache location It refersto the place at where the cached data is stored. Let’s assume original location of a file is on its server’s disk. Three possible cache location: 1) Server’s main memory 2) Client’s disk 3) Client’s main memory
- 9. Possible cache locationsin file-caching scheme for DFS //Do not learn it...Just read it.....
- 10. Modification Propagation Write-through: when newuser modifies the cache entry it is immediately written to the server. Delayed-write: To decrease continuous network traffic write all updates to the server periodically or batch them together.
- 11. File replication Provide forincrease reliability by having independent backups of each file. Enable file access to continue even if one file server is down.
- 12. Advantages of replication Improverperformance Increase reliability High Availability.
- 13. Replica creation methods Threemethods a) Explicit file replication b) Lazy file replication c) File replication using group
- 14. Explicit file replication Entireprocess controlled by programmer Process makes file on one server & Make multiple copies on servers. Directory server maintain list of all replicas When file is requested, any one of these copies can be opened.
- 15. Lazy file replication Onlyone copy created on server & server makes replication for other servers. System can track all replicas & retrieve one copy as required.
- 16. File replication usinga group System call is sent to all servers Replicas are created when original is made All copies are made at same time.
- 17. Fault Tolerance Stateful – Servermaintains information about a file opened by a client . (e.g…. file pointer, mode). – Mechanism: on open, the server provides a “handle” to the client to use on subsequent operations. Stateless – Server maintains no information about client access to files – Mechanism: each client operation must provide context information for that operation.
- 18. Stateful service Information about file operations are kept in the server during all the file session. A communication channel is established between the client and the server when a the client explicitly open a file. A number(identifier) is used to define the communication channel then this identifier will be used to perform file operations. To attend its clients, the server copies data from the storage devices to memory and let them there till the file closing.
- 19. Stateless service The stateless service does not establish a communication channel. there is no necessity for explicit file opening and closing Before executing a file operation the server will automatically open and close the file. Each request sent to the server must define the desired file likewise, if a read or write Operation is requested, it must contain the position in the file referring to the respective operation.
- 20. NFS • Sun NetworkFile System (NFS) has become de facto standard for distributed UNIX file access. • NFS runs over LAN – even WAN (slowly). • Any system may be both a client and server • Basic idea: – Remote directory is mounted onto local directory – Remote directory may contain mounted directories within
- 21. Andrew File System(AFS) Completely different kind of file system Developed at CMU to support all student computing. Consists of workstation clients and dedicated file server machines. • Single name space – File has the same names everywhere in the world. • Lots of local file caching – On workstation disks – For long periods of time – Originally whole files, now 64K file chunks. • Good for distant operation because of local disk caching 43