Distributed File Systems
- 1. Distributed File Systems File Caching Schemes File Replication Fault Tolerance Presented by: Maurvi Dave (T1013CSE2023) Pooja Upadhyay(T1013CSE2028)
- 2. File caching schemes Contents Introduction Key decisions of file caching schemes Cache location Modification propagation
- 3. Introduction To Retain recently accessed files in the main memory. Repeated accesses to the same information can be handled locally. Reduce network traffic
- 4. Key decisions In implementation of file caching scheme, one has to make several key decisions Like granularity of cached data Cache size Replacement policy
- 5. Cache location It refers to 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
- 6. Possible cache locations in file- caching scheme for DFS
- 7. Modification Propagation Write-through: when new user 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. Single write operation is efficient then multiple
- 8. File replication Provide for increase reliability by having independent backups of each file. Enable file access to continue even if one file server is down.
- 9. Advantages of replication Improver performance By distributing workload among multiple servers Increase reliability Copy can be taken from another server if one crashes Availability By multiple copy of each file on separated servers
- 10. Key issue in replication Replication Transparency Not transparent to the user. Entire process is carry behind programmer’s back.
- 11. Replica creation methods Three methods a) Explicit file replication b) Lazy file replication c) File replication using group
- 12. Explicit file replication Entire process 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.
- 13. Lazy file replication Only one copy created on server & server makes replication for other servers. System can track all replicas & retrieve one copy as required.
- 14. File replication using a group System call is sent to all servers Replicas are created when original is made All copies are made at same time.
- 15. Stateful – Server maintains 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 Fault Tolerance
- 16. 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 solicits the file opening. 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.
- 17. 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.