Distributed File Systems Group A5 Amit Sharma Dhaval Sanghvi Ali Abbas Outline What is a DFS Requirements of a DFS Sun Network File System History Architecture Protocols Implementation Basics File named collection of logically related data Unix file an uninterpreted sequence of bytes File system Provides a logical view of data and storage functions User friendly interface Provides facility to create modify organize and delete files Provides sharing among users in a controlled manner Provides protection What is a DFS A distributed implementation of time sharing model of a file system where multiple users share files and storage resources Overall storage space managed by a DFS consists of different remotely located smaller storage spaces Requirements Transparency Access transparency Location transparency Mobility transparency Failure transparency Performance transparency Other Requirements Scaling Security Hardware and operating System heterogeneity Sun s Network File System Introduced by Sun Microsystems in 1985 Sun published the protocol and licensed reference implementation Since then NFS has been supported by every Unix variant NFS design objectives Machine and OS independence no recompilation of applications Crash recovery Transparent access Reasonable performance comparable to local FS NFS The basic idea allow an arbitrary collection of clients and servers to share a common file system In most cases all clients and servers are on the same LAN each machine can be both a client and a server Each NFS server exports one or more of its directories for access by remote clients NFS The basic idea cont When a directory is made available so are all of its subdirectories whole directory trees are exported by NFS as a unit The list of exported directories a server exports is maintained in the etc exports file Uses RPC XDR NFS How do we get the files Mount protocol access shared file systems by mounting them from an NFS server machine Where at mount point Mount point an empty directory or subdirectory created as place to attach a remote file system How do we get the files cont server returns a file handle to the client The file handle contains fields uniquely identifying the file system type ext2 vfat Novell BSD NeXTSTEP the disk the i node number of the directory and security information How do we get the files cont The server daemons nfsd The NFS Daemon which services requests from NFS clients mountd The NFS Mount Daemon which actually carries out requests that nfsd passes on to it portmap The portmapper daemon which allows NFS clients to find out which port the NFS server is using NFS software architecture VFS VFS allows diverse specific file systems to coexist in a file tree isolating all FSdependencies in pluggable filesystem modules VFS was an internal kernel restructuring with no effect on the syscall interface VFS layer maintains a table with one entry for each open file VFS 2 VFS layer has an entry called a v node virtual i node for every open file V nodes are used to tell whether the file is local or remote A V node points to either an i node when the file is on the local disk or an r node in the NFS Client code when the reference is to data on a remote disk all state information on the open files is stored on the client s side Vnode use To mount a remote file system the system admn or etc rc calls the mount program Kernel constructs vnode for remote directory and asks NFS client code to create an r node in its internal tables Vnode in client VFS will point to local Inode or r node NFS implementation Servers are stateless Each request has complete information does not rely on previous state i e idempotent User s identity must be verified for each request Most UNIX system calls are supported except for open and close Idempotent idem po tent Pronunciation I d m pO t nt Date 1870 relating to or being a mathematical quantity which when applied to itself under a given binary operation as multiplication equals itself also relating to or being an operation under which a mathematical quantity is idempotent Semantics of file sharing On a single processor when a read follows a write the value returned by the read is the value just written In a distributed system with caching obsolete values may be returned Semantics of file sharing NFS implements session semantics Method Comment UNIX semantics Every operation on a file is instantly visible to all processes Session semantics No changes are visible to other processes until the file is closed Immutable files No updates are possible simplifies sharing and replication Transaction All changes occur atomically Caching The cache consistency problem cached data may become stale if cached data is updated elsewhere in the network NFS solution Timestamp invalidation Timestamp each cache entry and periodically query the server has this file changed since time t invalidate cache if stale NFS Client Caching Where in main memory of clients What file blocks translation of file names to vnodes and attributes of files and directories 1 File blocks time stamp of file when last modified on the server After certain age blocks have to be validated with server delay writing policy modified blocks flushed to server after certain delay NFS Client Caching Clients do not free delayed write blocks until the server confirms that the data have been written to disk 2 Caching of file names to vnodes for remote directory access speeds up the lookup procedure 3 Caching of file and directory attributes updated when new attributes received from server discarded after certain time NFS Client Caching Writes block marked dirty and scheduled for flushing flushing when file is closed or a sync occurs at client What if multiple clients write to same file at the same time Can get either version or parts of both Completely arbitrary Just like normal Unix Problem Writes from clients So if writes happen at time t and close happens at t then other clients might not see new data till t Cache validation Validation check performed at file open whenever server contacted to get new block after timeout 3s for file blocks 30s for directories Done for all files even if not being shared Expensive Potentially every 3 sec get file attributes If needed invalidate all blocks Locking in NFS Operation Description Lock Creates a lock for a range of bytes non blocking Lockt Test whether a conflicting lock has been granted Locku Remove a lock from a range of bytes Renew Renew the lease on a specified