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CS 5523 Lecture 22:
Distributed File Systems
❚ Requirements
❚ Storage system support infrastructure
❚ How does Unix do it?
❚ Flat file services
❚ Directory structure
❚ Introduction to NFS (Network File System)
Distributed file system requirements:
❚ Transparency
❙ access transparency
❙ location transparency
❙ mobility transparency
❙ performance transparency
❙ scaling transparency
❚ Concurrency control possibility with support for transactions
❚ Replication
❚ Fault tolerance
❚ Heterogeneity
❚ Security
❚ Consistency
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 8.
Storage systems and their properties
Sharing Persis- tence
Distributed cache/replicas
Consistency maintenance
Example
Main memory (^) RAM
File system UNIX file system
Distributed file system Sun NFS
Web Web server
Distributed shared memory Ivy (Ch. 16)
Remote objects (RMI/ORB) CORBA
Persistent object store 1 CORBA Persistent Object Service
Persistent distributed object store PerDiS, Khazana
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
File system modules
Directory module: relates file names to file IDs
File module: relates file IDs to particular files
Access control module: checks permission for operation requested
File access module: reads or writes file data or attributes
Block module: accesses and allocates disk blocks
Device module: disk I/O and buffering
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 8.
File attribute record structure
File length
Creation timestamp
Read timestamp
Write timestamp
Attribute timestamp
Reference count
Owner
File type
Access control list
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 8.
UNIX file system operations
filedes = open(name, mode) filedes = creat(name, mode)
Opens an existing file with the given name. 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) count = write(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. Both operations deliver the number of bytes actually transferred and advance the read-write pointer.
pos = lseek(filedes, offset, whence)
Moves the read-write pointer to offset (relative or absolute, 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.
Proposed flat file service versus Unix:
❚ No open or close
❚ Most operations (except create) are idempotent (UNIX read and
write are not)
❚ Access control is different:
❙ In UNIX access rights are checked at the open. Process uses a file handle.
❙ In the proposed service access rights are checked by the server:
❘ Either when a name is converted to UFID access rights are encoded in a capability. The capability is then presented with each subsequent operations
❘ Or, the user credentials are submitted with each request
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 8.
Directory service operations
Lookup(Dir, Name) -> FileId — throws NotFound
Locates the text name in the directory and returns the relevant UFID. If Name is not in the directory, throws an exception.
AddName(Dir, Name, File) — throws NameDuplicate
If Name is not in the directory, adds ( Name , File ) to the directory and updates the file’s attribute record. If Name is already in the directory: throws an exception.
UnName(Dir, Name) — throws NotFound
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.
Proposed directory system versus Unix:
❚ Text names are translated to UIDs
❚ UNIX file names are mapped to inodes in a specific device
partition
❚ Hierarchical organization can be supported in both (need a type
designation)
❚ File groups used to allocate blocks of files in a distributed file
system to particular servers (analogous to a filesystem in UNIX)
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
NFS architecture
UNIX kernel
protocol
Client computer Server computer
system calls
Local Remote
UNIX
file system
NFS
client
NFS
server
UNIX
file system
Application program
Application program
NFS
UNIX
UNIX kernel
Virtual file system Virtual file system
Otherfile system
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 8.
NFS server operations (simplified) – 1
lookup(dirfh, name) -> fh, attr (^) Returns file handle and attributes for the file name in the directory dirfh. create(dirfh, name, attr) -> newfh, attr
Creates a new file name in directory dirfh with attributes attr and returns the new file handle and attributes. remove(dirfh, name) status (^) Removes file name from directory dirfh.
getattr(fh) -> attr (^) Returns file attributes of file fh. (Similar to the UNIX stat system call.) setattr(fh, attr) -> attr Sets the attributes (mode, user id, group id, size, access time and modify time of a file). Setting the size to 0 truncates the file. read(fh, offset, count) -> attr, data (^) Returns up to count bytes of data from a file starting at offset. Also returns the latest attributes of the file. write(fh, offset, count, data) -> attr (^) Writes count bytes of data to a file starting at offset. Returns the attributes of the file after the write has taken place. rename(dirfh, name, todirfh, toname) -> status
Changes the name of file name in directory dirfh to toname in directory to todirfh.
link(newdirfh, newname, dirfh, name) -> status
Creates an entry newname in the directory newdirfh which refers to file name in the directory dirfh. Continues on next slide ...
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 8.
NFS server operations (simplified) – 2
symlink(newdirfh, newname, string) -> status
Creates an entry newname in the directory newdirfh of type symbolic link with the value string. The server does not interpret the string but makes a symbolic link file to hold it. readlink(fh) -> string (^) Returns the string that is associated with the symbolic link file identified by fh.
mkdir(dirfh, name, attr) -> newfh, attr
Creates a new directory name with attributes attr and returns the new file handle and attributes.
rmdir(dirfh, name) -> status Removes the empty directory name from the parent directory dirfh. Fails if the directory is not empty.
readdir(dirfh, cookie, count) -> entries
Returns up to count bytes of directory entries from the directory dirfh. Each entry contains a file name, a file handle, and an opaque pointer to the next directory entry, called a cookie. The cookie is used in subsequent readdir calls to start reading from the following entry. If the value of cookie is 0, reads from the first entry in the directory.
statfs(fh) -> fsstats Returns file system information (such as block size, number of free blocks and so on) for the file system containing a file fh.
