1. File System Interface and Implementations
Fred Kuhns
CS523 – Operating Systems
Fred Kuhns ()
CS523S: Operating Systems

2. CS523S: Operating Systems
FS Framework in UNIX
Provides persistent storage
Facilities for managing data
file - abstraction for data container, supports sequential and random access
file system - permits organizing, manipulating and accessing files
User interface specifies behavior and semantics of relevant system calls
Interface exported abstractions: files, directories, file descriptors and different file systems
Fred Kuhns ()
CS523S: Operating Systems

3. Kernel, Files and Directories
kernel provides control operations to name, organize and control access to files but it does not interpret contents
Running programs have an associated current working directory. Permits use of relative pathnames. Otherwise complete pathnames are required.
File viewed as a collection of bytes
Applications requiring more structure must define and implement themselves
Fred Kuhns ()
CS523S: Operating Systems

4. Kernel, Files and Directories
files and directories form hierarchical tree structure name space.
tree forms a directed acyclic graph
Directory entry for a file is known as a hard link.
Files may also have symbolic links
File may have one or more links
POSIX defines library routines {opendir(), readdir(), rewinddir(), closedir()}
struct dirent {
ino_t d_ino;
char d_name[NAME_MAX + 1]; }
Fred Kuhns ()
CS523S: Operating Systems

5. File and Directory Organization
(hard) links /
bin
etc
dev
usr
vmunix sh
local
etc
/usr/local/bin/bash
bin
bash
Fred Kuhns ()
CS523S: Operating Systems

6. CS523S: Operating Systems
File Attributes
Type – directory, regular file, FIFO, symbolic link, special.
Reference count – number of hard links {link(), unlink()}
size in bytes
device id – device files resides on
inode number - one inode per file, inodes are unique within a disk partition (device id)
ownership - user and group id {chown()}
access modes - Permissions and modes {chmod()}
{read, write execute} for {owner, group or other}
timestamps – three different timestamps: last access, last modify, last attributes modified. {utime()}
Fred Kuhns ()
CS523S: Operating Systems

7. CS523S: Operating Systems
Permissions and Modes
Three Mode Flags = {suid, sgid and sticky}
suid –
File: if set and executable then set the user’s effective user id
Directory: Not used
sgid –
File: if set and executable then set the effective group id. If sgid is set but not executable then mandatory file/record locking
Directory: if set then new files inherit group of directory otherwise group or creator.
sticky –
File: if set and executable file then keep copy of program in swap area.
Directory: if set and directory writable then remove/rename if EUID = owner of file/directory or if process has write permission for file. Otherwise any process with write permission to directory may remove or rename.
Fred Kuhns ()
CS523S: Operating Systems

8. CS523S: Operating Systems
User View of Files
File Descriptors (open, dup, dup2, fork)
All I/O is through file descriptors
references the open file object
per process object
file descriptors may be dup’ed {dup(), dup2()}, copied on fork {fork()} or passed to unrelated process {(see ioctl() or sendmsg(), recvmsg()}permitting multiple descriptors to reference one object.
File Object - holds context
created by an open() system call
stores file offset
reference to vnode
vnode - abstract representation of a file
Fred Kuhns ()
CS523S: Operating Systems

9. How it works fd = open(path, oflag, mode);
lseek(), read(), write() affect offset
File Descriptors
{{0, uf_ofile}
{1, uf_ofile}
{2 , uf_ofile}
{3 , uf_ofile}
{4 , uf_ofile}
{5 , uf_ofile}}
Open File Objects
{*f_vnode,f_offset,f_count,...},
{*f_vnode,f_offset,f_count,...}}
Vnode/vfs
In-memory
representation
of file
Vnode/vfs
In-memory
representation
of file
Vnode/vfs
In-memory
representation
of file
Vnode/vfs
In-memory
representation
of file
Vnode/vfs
In-memory
representation
of file
Fred Kuhns ()
CS523S: Operating Systems

10. CS523S: Operating Systems
File Systems
File hierarchy composed of one or more File Systems
One File System is designated the Root File System
Attached to mount points
File can not span multiple File Systems
Resides on one logical disk
Fred Kuhns ()
CS523S: Operating Systems

11. CS523S: Operating Systems
Logical Disks
Viewed as linear sequence of fixed sized, randomly accessible blocks.
device driver maps FS blocks to underlying storage device.
created using newfs or mkfs utilities
A file system must reside in a logical disk, however a logical disk need not contain a file system (for example the swap device).
Typically logical disk corresponds to partion of a physical disk. However, logical disk may:
map to multiple physical disks
be mirrored on several physical disks
striped across multiple disks or other RAID techniques.
Fred Kuhns ()
CS523S: Operating Systems

12. CS523S: Operating Systems
File Abstraction
Abstracts different types of I/O objects
for example directories, symbolic links, disks, terminals, printers, and pseudodevices (memory, pipes sockets etc).
Control interface includes fstat, ioctl, fcntl
Symbolic links: file contains a pathname to the linked file/directory. {lstat(), symlink(), readlink()}
Pipe and FIFO files:
FIFO created using mknod(), lives in the file system name space
Pipe created using pipe(), persists as long as opened for reading or writing.
Fred Kuhns ()
CS523S: Operating Systems

13. CS523S: Operating Systems
OO Style Interfaces
Abstract base class
Instance of derived class
Struct interface_t {
// Common functions:
open (), close ()
// Common data:
type, count
// Pure virtual functions
*ops (Null pointer)
// Private data
*data (Null pointer) }
Struct interface_t {
open (), close ()
type, count
*ops
*data }
{my_read()
my_write()
my_init()
my_open()
… }
{device_no,
free_list,
lock, …}
Fred Kuhns ()
CS523S: Operating Systems

14. CS523S: Operating Systems
Overview
System calls
vnode interface
tmpfs
swapfs
UFS
HSFS
PCFS
RFS
/proc
NFS
disk
cdrom
diskette
Process
address
space
Anonymous
memory
Example from Solaris
Fred Kuhns ()
CS523S: Operating Systems

15. CS523S: Operating Systems
Vfs/Vnode Framework
Concurrently support multiple file system types
transparent interoperation of different file systems within one file hierarchy
enable file sharing over network
abstract interface allowing easy integration of new file systems by vendors
Fred Kuhns ()
CS523S: Operating Systems

16. CS523S: Operating Systems
Objectives
Operation performed on behalf of current process
Support serialized access, I.e. locking
must be stateless
must be reentrant
encourage use of global resources (cache, buffer)
support client server architectures
use dynamic storage allocation
Fred Kuhns ()
CS523S: Operating Systems

17. CS523S: Operating Systems
Vnode/vfs interface
Define abstract interfaces
vfs: Fundamental abstraction representing a file system to the kernel
Contains pointerss to file system (vfs) dependent operations such as mount, unmount.
vnode: Fundamental abstraction representing a file in the kernel
defines interface to the file, pointer to file system specific routines. Reference counted.
accessed in two ways:
1) I/O related system calls
2) pathname traversal
Fred Kuhns ()
CS523S: Operating Systems

18. CS523S: Operating Systems
vfs Overview
Struct vfsops {
*vfs_mount,
*vfs_root, …}
Struct vfsops {
*vfs_mount,
*vfs_root, …}
rootvfs
private data
private data
Struct vfs {
*vfs_next,
*vfs_vnodecovered,
*vfs_ops,
*vfs_data, …}
Struct vfs {
*vfs_next,
*vfs_vnodecovered,
*vfs_ops,
*vfs_data, …}
Struct vnode {
*v_vfsp,
*v_vfsmountedhere,…}
Struct vnode {
*v_vfsp,
*v_vfsmountedhere,…}
Struct vnode {
*v_vfsp,
*v_vfsmountedhere,…}
/ (root)
/usr
/ (mounted fs)
Fred Kuhns ()
CS523S: Operating Systems

19. CS523S: Operating Systems
Mounting a FS
mount(spec, dir, flags, type, dataptr, datalen);
SVR5 uses a global virtual file system switch table (vfssw)
allocate and initialize private data
initialize vfs struct
initialize root vnode in memory (VFS_ROOT)
Fred Kuhns ()
CS523S: Operating Systems

20. CS523S: Operating Systems
Pathname traversal
Verify vnode is dir or stop
invoke VOP_LOOKUP (ufs_lookup())
if found, return pointer to vnode (locked)
else not found and last component, return success and vnode of parent directory (locked)
not found, release directory, repeat loop
Fred Kuhns ()
CS523S: Operating Systems

21. CS523S: Operating Systems
Local File Systems
S5fs - System V file system. Based on the original implementation.
FFS/UFS - BSD developed filesystem with optimized disk usage algorithms
Fred Kuhns ()
CS523S: Operating Systems

22. CS523S: Operating Systems
S5fs - Disk layout
Viewed as a linear array of blocks
Typical disk block size 512, 1024, 2048 bytes
Physical block number is the block’s index
disk uses cylinder, track and sector
first few blocks are the boot area, which is followed by the inode list (fixed size)
Fred Kuhns ()
CS523S: Operating Systems

23. CS523S: Operating Systems
Disk Layout
tract
sector
heads
cylinder
platters
Rotational speed
disk seek time
Fred Kuhns ()
CS523S: Operating Systems

24. CS523S: Operating Systems
S5fs disk layout
bootarea
superblock
inode list
data
Boot area - code to initialize bootstrap the system
Superblock - metadata for filesystem. Size of FS, size
of inode list, number of free blocks/inodes,
free block/inode list
inode list - linear array of 64byte inode structs
Fred Kuhns ()
CS523S: Operating Systems

25. CS523S: Operating Systems
s5fs - some details
inode
name
Di_mode (2)
di_nlinks (2)
di_uid (2)
di_gid (2)
di_size (4)
di_addr (39)
di_gen (1)
di_atime (4)
di_mtime (4)
di_ctime (4) 8 . 45 .. “”
123
myfile
2 byte
14byte
directory
On-disk inode
Fred Kuhns ()
CS523S: Operating Systems

26. Locating file data blocks
Assume 1024 Byte Blocks 1 2 3 4 5 6 7 8 9
10 - indirect
11 - double indirect
12 - triple indirect
256 blocks
65,536 blocks
16,777,216 blocks
Fred Kuhns ()
CS523S: Operating Systems

27. S5fs Kernel Implementation
In-Core Inodes - also include vnode, device id, inode number, flags
Inode lookup uses a hash queue based on inode number (amy also use device number)
kernel locks inode for reading/writing
Read/Write use a buffer cache or VM
Fred Kuhns ()
CS523S: Operating Systems

28. CS523S: Operating Systems
Problems with s5fs
Superblock
on-disk inodes
Disk block allocation
file name size
Fred Kuhns ()
CS523S: Operating Systems

29. CS523S: Operating Systems
Fast File System - FFS
Disk partition divided into cylinder groups
superblocks restructured and replicated across partition
Constant information
cylinder group summary info such as free inodes and free block
support block fragments
Long file names
new disk block allocation strategy
Fred Kuhns ()
CS523S: Operating Systems

30. FFS Allocation strategy
Goal: Collocate similar data/info.
file inodes located in same cyl group as dir.
new dirs created in different cyl groups.
Place file data blocks/inode in same cyl group - for size < 48K
allocate sequential blocks at a rotationally optimal position.
Choose cyl group with “best” free count
Fred Kuhns ()
CS523S: Operating Systems

31. CS523S: Operating Systems
Is FFS/UFS Better?
Measurements have shown substantial performance benefits over s5fs
FFS however, is sub-optimal when the disk is nearly full. Thus 10% is always kept free.
Modern disks however, no longer match the underlying assumptions of FFS
Fred Kuhns ()
CS523S: Operating Systems

32. CS523S: Operating Systems
Buffer Cache
Free
(LRU)
Hash (device,inode)
Fred Kuhns ()
CS523S: Operating Systems

33. Other Limitations of s5fs and FFS
Performance - hardware designs and modern architectures have redefined the computing environment
Crash Recovery do you like waiting for fsck()?
Security - do we need more than just 7 bits
File Size limitations
Fred Kuhns ()
CS523S: Operating Systems

34. CS523S: Operating Systems
Performance Issues
FFS has a target rotational delay
read/write entire track
many disks have built-in caches
Due to FS Caching, most I/O operations are writes.
Synchronous writes of metadata
Disk head seeks are expensive
Fred Kuhns ()
CS523S: Operating Systems

35. CS523S: Operating Systems
Sun-FFS (cluster)
Sets rotational delay to 0
read clustering
write clustering
Fred Kuhns ()
CS523S: Operating Systems

36. CS523S: Operating Systems
Log-Structured FS
Entire disk dedicated to log
writes to tail of log file
garbage collection daemon
Dir and Inode structures retained
Issue is locating inodes
writes a segment at a time
Fred Kuhns ()
CS523S: Operating Systems

37. CS523S: Operating Systems
Log-structured FS
Requires a large cache for read efficiency
Write efficiency is obtained since the system is always writing to the end of the log file. Why does this help?
Why does performance compare to Sun-FFS?
What about crash recovery?
Fred Kuhns ()
CS523S: Operating Systems

38. CS523S: Operating Systems
4.4BSD Portal FS
Portal
daemon
User process
/p/<path>
<path> fd fd
Protal file system
Sockets
Fred Kuhns ()
CS523S: Operating Systems

39. CS523S: Operating Systems
Review of vnode/vfs
Provides a general purpose interface
allows multiple file systems to be used simultaneously in a system
OO Interface -
although limited, no inheritance, fixed interfaces
How can we improve on this?
Fred Kuhns ()
CS523S: Operating Systems

40. Stackable Filesystems
application
application
/mylocal
MyFS
/local
UFS
For a given mount point, there is now possible many file systems
Fred Kuhns ()
CS523S: Operating Systems