DISTRIBUTED FILE SYSTEMS

DISTRIBUTED FILE SYSTEMS From Chapter 8 of Distributed Systems Concepts and Design,4th Edition, By G. Coulouris, J. Dollimore and T. Kindberg Published by Addison Wesley/Pearson Education June 2005

Topics Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

Introduction Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

Sharing Persis- Distributed Consistency Example tence cache/replicas maintenance Main memory 1 RAM 1 File system UNIX file system Distributed file system Sun NFS Web server Web Distributed shared memory Ivy (Ch. 18) Remote objects (RMI/ORB) CORBA 1 1 Persistent object store CORBA Persistent Object Service Peer-to-peer storage system OceanStore(Ch. 10) Introduction Figure 1. Storage systems and their properties Types of consistency between copies: 1 - strict one-copy consistency √ - approximate consistency X - no automatic consistency Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

Introduction Figure 2.File system modules Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

File length Creation timestamp Read timestamp Write timestamp Attribute timestamp Reference count Owner File type Access control list Introduction Figure 3.File attribute record structure Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

Figure 4. UNIX file system operations filedes = open(name, mode) Opens an existing file with the given name. filedes = creat(name, mode) Creates a new file with the given name. Both operations deliver a file descriptor referencing the open file. The mode is read, write or both. status = close(filedes) Closes the open file filedes. count = read(filedes, buffer, n) Transfers n bytes from the file referenced by filedes to buffer. Transfers n bytes to the file referenced by filedes from buffer. count = write(filedes, buffer, n) Both operations deliver the number of bytes actually transferred and advance the read-write pointer. pos = lseek(filedes, offset, Moves the read-write pointer to offset (relative or absolute, whence) depending on whence). status = unlink(name) Removes the file name from the directory structure. If the file has no other names, it is deleted. status = link(name1, name2) Adds a new name (name2) for a file (name1). status = stat(name, buffer) Gets the file attributes for file name into buffer. Introduction Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

File Service Architecture Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

Client computer Server computer Directory service Application Application program program Flat file service Client module File Service Architecture Figure 5.File service architecture Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

File Service Architecture Read(FileId, i, n) -> Data if 1≤i≤Length(File): Reads a sequence of up to n items -throws BadPosition from a file starting at item i and returns it in Data. Write(FileId, i, Data) if 1≤i≤Length(File)+1: Write a sequence of Data to a -throws BadPosition file, starting at item i, extending the file if necessary. Create() -> FileId Creates a new file of length0 and delivers a UFID for it. Delete(FileId) Removes the file from the file store. GetAttributes(FileId) -> Attr Returns the file attributes for the file. SetAttributes(FileId, Attr) Sets the file attributes (only those attributes that are not shaded in Figure 3.) Figure 6.Flat file service operations Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

File Service Architecture Lookup(Dir, Name) -> FileId Locates the text name in the directory and -throws NotFound returns the relevant UFID. If Name is not in the directory, throws an exception. AddName(Dir, Name, File) If Name is not in the directory, adds(Name,File) -throws NameDuplicate to the directory and updates the file’s attribute record. If Name is already in the directory: throws an exception. UnName(Dir, Name) If Name is in the directory, the entry containing Name is removed from the directory. If Name is not in the directory: throws an exception. GetNames(Dir, Pattern) -> NameSeq Returns all the text names in the directory that match the regular expression Pattern. Figure 7.Directory service operations Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

To construct a globally unique ID we use some unique attribute of the machine on which it is created, e.g. IP number, even though the file group may move subsequently. File Group ID: 32 bits 16 bits IP address date File Service Architecture Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

DFS: Case Studies Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

Case Study: Sun NFS Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

UNIX system calls UNIX kernel Operationson remote files Operations on local files Other file system NFS protocol (remote operations) NFS architecture Client computer Server computer Application Application program program UNIX kernel Virtual file system Virtual file system UNIX UNIX NFS NFS file file client server system system Figure 8.NFSarchitecture Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 *

fh = file handle: Filesystem identifier i-node number i-node generation Case Study: Sun NFS Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

Case Study: Sun NFS • read(fh, offset, count) -> attr, data • write(fh, offset, count, data) -> attr • create(dirfh, name, attr) -> newfh, attr • remove(dirfh, name) status • getattr(fh) -> attr • setattr(fh, attr) -> attr • lookup(dirfh, name) -> fh, attr • rename(dirfh, name, todirfh, toname) • link(newdirfh, newname, dirfh, name) • readdir(dirfh, cookie, count) -> entries • symlink(newdirfh, newname, string) -> status • readlink(fh) -> string • mkdir(dirfh, name, attr) -> newfh, attr • rmdir(dirfh, name) -> status • statfs(fh) -> fsstats Figure 9.NFSserver operations (NFS Version 3 protocol, simplified) Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

Case Study: Sun NFS Note: The file system mounted at /usr/students in the client is actually the sub-tree located at /export/people in Server 1; the file system mounted at /usr/staff in the client is actually the sub-tree located at /nfs/users in Server 2. Figure 10.Local and remote file systems accessible on an NFS client Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

Case Study: Sun NFS

Case Study: Sun NFS t freshness guarantee Tc time when cache entry was last validated Tm time when block was last updated at server T current time

Case Study: Sun NFS

Case Study: The Andrew File System (AFS) Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

Case Study: The Andrew File System (AFS) Figure 11. Distribution of processes in the Andrew File System Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

Case Study: The Andrew File System (AFS) Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

Case Study: The Andrew File System (AFS) Figure 12. File name space seen by clients of AFS Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

DISTRIBUTED FILE SYSTEMS