1 overview assignment 12: hints distributed file systems assignment 11: solution file systems
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Overview Assignment 12: hints
Distributed file systems Assignment 11: solution
File systems
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A12 – Network File System (Sun)
Virtual file system layer
System call layer
Local operatingsystem
NFS client
Messageto server
Virtual file system layer
Local operatingsystem
NFS server
Messagefrom client
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A12 – NFS Mounting protocol:
The server exports certain directories to certain machines (IP authentication)
The clients mounts a given directory from a specified server.
Directory and file access: System calls are performed as remote procedure
calls. No open and close: stateless.
Security: IP based (export lists) UNIX permissions based on UID/GID
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A12 – Andrew File System (CMU)
Virtual file system layer
System call layer
Local operatingsystem
AFS client
Messageto server
Virtual file system layer
Local operatingsystem
AFS server
Messagefrom clientAFS
cache
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A12 – AFS Mounting
The server has no information about the clients. Cell list maintained. The client mounts the whole AFS tree (the whole AFS
world).
Directory and file access The file is cached locally and committed on close.
Security: Kerberos server Access list for files access
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Server B
A12 Ex1 – Moving a shared volume
Client
Server A
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A12 Ex2 – Simultaneous accesses
Client
• open• modify• close
Server
Client
• open• modify• close
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A12 Ex3 – Immutable Files Files can only be created and read.
What is simplified?What can be avoided?
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Overview Assignment 12: hints
Distributed file systems Assignment 11: solution
File systems
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A11 Ex1 – Max File Length datablocks
inode
direct 0
direct 1
direct 2
direct 3
direct 4
direct 5
direct 6
direct 7
direct …
direct 11single indirect
double indirect
triple indirect
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A11 Ex1 – Maximal File LengthGiven: block size = 2KB address = 4 bytes (assumption) table of contents = 15 entries (12 / 1 / 1 / 1)
number of entries in index table = 2048 / 4 = 512
GB256B382
)3512125121512112(KB2
Length
Length
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A11 Ex2 – File Access./a/b/c/f, File length is 1GB
Directory access 2 disk acc. (1 for the inode, 1 for table) 7 accesses until the file inode is knownPositioning at the end of the file (2K blocks): direct access: 24 KB single indirect access: 1 MB double indirect access: 512 MB must use triple indirect access 4 accesses (1i+3t)Read the block and rewrite it with the additional byte
2 accesses
Total: 13 disk accesses
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A11 Ex3 – I/O redirection / Pipes
$ command < file
After shell fork() in the child process: fd = open(file) close(stdin) dup2(stdin,fd) close(fd) exec(command)
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A11 Ex3 – I/O redirection / Pipes
0 stdin
1 stdout
2 stderr
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… …
count 2, wr
user file descriptor file table
cnt 2 (/dev/tty1)
inode table
count 1 rd cnt 1 (file)
$ command < file
fd = open(file) close(stdin)dup2(stdin,fd)close(fd)exec(command)
count 1, rd
count 2, rd
cnt 1 (/dev/tty1)
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A11 Ex3 – I/O redirection / Pipes
$ command1 | command2
Shell
fork()pipe(fd[])fork()
close(stdout)dup2(stdout,fd[1])close(fd[1])exec(command1)
close(stdin)dup2(stdin,fd[0])close(fd[0])exec(command2)
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A11 Ex4 – MountingBach Book:
Each file system type has a method tableTo access a file, indirect function call through
the table
need FS abstraction, common to all FS struct file_operation read, write, readdir, ioctl, open, flush, release, fsync,
lock associate every file with its FS (method table)
inode has a pointer to file_operation