File System

Raju Pandey
Department of Computer Sciences
University of California, Davis
Spring 2011
 Overall view

Application
Program

mount()
write() WriteFile()
CreateFile()
close() open() CloseHandle() ReadFile()
lseek() read()
SetFilePointer()

Memory Mgr

Memory Mgr
Process Mgr

Process Mgr
Device Mgr

Device Mgr
File Mgr

File Mgr
UNIX Windows

Hardware

ECS 150 (Operating Systems) File System (1), 2 Spring 2011 UC Davis
 Files
• A file is a collection of data with some properties
ƒ contents, size, owner, last read/write time, protection …
• Files may also have types
ƒ understood by file system
o device, directory, symbolic link
ƒ understood by other parts of OS or by runtime libraries
o executable, dll, source code, object code, text file, …

• Type can be encoded in the file’s name or contents

ƒ windows encodes type in name
o .com, .exe, .bat, .dll, .jpg, .mov, .mp3, …
ƒ old Mac OS stored the name of the creating program along with
the file
ƒ unix has a smattering of both
o in content via magic numbers or initial characters (e.g., #!)

ECS 150 (Operating Systems) File System (1), 3 Source: Gribble, Lazowska,
Levy, Zahorjan
 File Management
Applications
• The file manager
Records
administers the
collection by:
ƒ Storing the information
on a device Structured Record Files

ƒ Mapping the block

Record-Stream Translation
storage to a logical view
ƒ Allocating/deallocating Byte Stream Files
storage
Stream-Block Translation
ƒ Providing file directories
• What abstraction
should be presented to Storage device

programmer?
ECS 150 (Operating Systems) File System (1), 4 Spring 2011 UC Davis
 Interface Layers

Std.
App. Disk
Runtime OS
Code
Library

Procedure Device-type
Calls Dependent Commands

Whatever… Syscalls

ECS 150 (Operating Systems) File System (1), 5 Source: Gribble, Lazowska,
Levy, Zahorjan
 Exported Abstractions

Std.
App. Disk
Runtime OS
Code
Library

OS + Directories, Array of Blocks

Whatever a tiny bit of Directory Entries,
file type / structure Files,…

/ /

root etc root etc

.xls

ECS 150 (Operating Systems) File System (1), 6 Source: Gribble, Lazowska,
Levy, Zahorjan
 Primary Roles of the OS (file system)

1. Hide hardware specific

interface
2. Allocate disk blocks
3. Check permissions
4. Understand directory
file structure OS Disk
5. Maintain metadata
6. Performance
7. Flexibility

Why does the OS define directories?

Why not leave that to the /

library/application layer? root etc

(Why would you want to leave it to

the app/library?)

ECS 150 (Operating Systems) File System (1), 7 Source: Gribble, Lazowska,
Levy, Zahorjan
 File access methods
• Some file systems provide different access methods that
specify ways the application will access data
ƒ sequential access
o read bytes one at a time, in order
ƒ direct access
o random access given a block/byte #
ƒ record access
o file is array of fixed- or variable-sized records
ƒ indexed access
o FS contains an index to a particular field of each record in a file
o apps can find a file based on value in that record (similar to DB)
• Why do we care about distinguishing sequential from direct
access?
ƒ what might the FS do differently in these cases?

ECS 150 (Operating Systems) File System (1), 8 Source: Gribble, Lazowska,
Levy, Zahorjan
 Byte Stream File Interface

fileID = open(fileName)
close(fileID)
read(fileID, buffer, length)
write(fileID, buffer, length)
seek(fileID, filePosition)

ECS 150 (Operating Systems) File System (1), 9 Spring 2011 UC Davis
 Low Level Files

fid = open(“fileName”,…);
…
read(fid, buf, buflen); b0 b1 b2 ... bi ...
…
close(fid);

int open(…) {…}

int close(…) {…}
int read(…) {…} Stream-Block Translation
int write(…) {…}
int seek(…) {…}

Storage device response to commands

ECS 150 (Operating Systems) File System (1), 10 Spring 2011 UC Davis
 Structured Files

Records

Record-Block Translation

ECS 150 (Operating Systems) File System (1), 11 Spring 2011 UC Davis
 Record-Oriented Sequential Files

Example: Mail system

Logical Record

fileID = open(fileName)
close(fileID)
getRecord(fileID, record)
putRecord(fileID, record)
seek(fileID, position)

ECS 150 (Operating Systems) File System (1), 12 Spring 2011 UC Davis
 Record-Oriented Sequential Files

Logical Record

H byte header k byte logical record

...

ECS 150 (Operating Systems) File System (1), 13 Spring 2011 UC Davis
 Indexed Sequential File

• Suppose we want to directly access records, rather than

sequentially
• Add an index to the file
• Each record includes an index field

fileID = open(fileName)
close(fileID)
getRecord(fileID, index)
index = putRecord(fileID, record)
deleteRecord(fileID, index)

ECS 150 (Operating Systems) File System (1), 14 Spring 2011 UC Davis
 Indexed Sequential File (cont)

Application structure
Account # Index index = i
012345
123456 i
294376 k
index = k
...
529366 j
...
965987 index = j

ECS 150 (Operating Systems) File System (1), 15 Spring 2011 UC Davis
 Directories
• Directories provide:
ƒ a way for users to organize their files
ƒ a convenient file name space for both users and FS’s
• Most file systems support multi-level directories
ƒ naming hierarchies (/, /usr, /usr/local, /usr/local/bin, …)
• Most file systems support the notion of current directory
ƒ absolute names: fully-qualified starting from root of FS
bash$ cd /usr/local

ƒ relative names: specified with respect to current directory

bash$ cd /usr/local (absolute)
bash$ cd bin (relative, equivalent to cd /usr/local/bin)

ECS 150 (Operating Systems) File System (1), 16 Source: Gribble, Lazowska,
Levy, Zahorjan
 Single-Level Directory

• A single directory for all users

Naming problem

Grouping problem

ECS 150 (Operating Systems) File System (1), 17 Source: Stallings

 Two-Level Directory

• Separate directory for each user

„ Path name
„ Can have the same file name for different user
„ Efficient searching
„ No grouping capability

ECS 150 (Operating Systems) File System (1), 18 Source: Stallings

 Tree-Structured Directories

ECS 150 (Operating Systems) File System (1), 19 Source: Stallings

 Tree-Structured Directories (Cont)
• Efficient searching

• Grouping Capability

• Current directory (working directory)

ƒ cd /spell/mail/prog
ƒ type list

ECS 150 (Operating Systems) File System (1), 20 Source: Stallings

 Tree-Structured Directories (Cont)

• Absolute or relative path name

• Creating a new file is done in current directory
• Delete a file
rm <file-name>
• Creating a new subdirectory is done in current
directory
mkdir <dir-name>
Example: if in current directory /mail
mkdir count

mail

prog copy prt exp count

Deleting “mail” ⇒ deleting the entire subtree rooted by

“mail”
ECS 150 (Operating Systems) File System (1), 21 Source: Stallings
 Acyclic-Graph Directories

• Have shared subdirectories and files

ECS 150 (Operating Systems) File System (1), 22 Source: Stallings

 Acyclic-Graph Directories (Cont.)
• Two different names (aliasing)

• If dict deletes list ⇒ dangling pointer

Solutions:
ƒ Backpointers, so we can delete all pointers
Variable size records a problem
ƒ Backpointers using a daisy chain organization
ƒ Entry-hold-count solution
• New directory entry type
ƒ Link – another name (pointer) to an existing file
ƒ Resolve the link – follow pointer to locate the file

ECS 150 (Operating Systems) File System (1), 23 Source: Stallings

 File System Mounting

• A file system must be mounted before it

can be accessed
• A file system is mounted at a mount
point (a directory)

ECS 150 (Operating Systems) File System (1), 24 Source: Stallings

 (a) Existing. (b) Unmounted Partition

ECS 150 (Operating Systems) File System (1), 25 Source: Stallings

 Mount Point

ECS 150 (Operating Systems) File System (1), 26 Source: Stallings

 Directory internals
• A directory is typically just a file that happens to contain
special metadata
ƒ directory = list of (name of file, file attributes)
ƒ attributes include such things as:
o size, protection, location on disk, creation time, access time, …
ƒ the directory list is usually unordered (effectively random)
o when you type “ls”, the “ls” command sorts the results for you

ECS 150 (Operating Systems) File System (1), 27 Source: Gribble, Lazowska,
Levy, Zahorjan
 Path name translation
• Let’s say you want to open “/one/two/three”
fd = open(“/one/two/three”, O_RDWR);
• What goes on inside the file system?
ƒ open directory “/” (well known, can always find)
ƒ search the directory for “one”, get location of “one”
ƒ open directory “one”, search for “two”, get location of “two”
ƒ open directory “two”, search for “three”, get loc. of “three”
ƒ open file “three”
ƒ (of course, permissions are checked at each step)
• FS spends lots of time walking down directory paths
ƒ this is why open is separate from read/write (session state)
ƒ OS will cache prefix lookups to enhance performance
o /a/b, /a/bb, /a/bbb all share the “/a” prefix

ECS 150 (Operating Systems) File System (1), 28 Source: Gribble, Lazowska,
Levy, Zahorjan
 File protection
• FS must implement some kind of protection system
ƒ to control who can access a file (user)
ƒ to control how they can access it (e.g., read, write, or exec)
• More generally:
ƒ generalize files to objects (the “what”)
ƒ generalize users to principals (the “who”, user or program)
ƒ generalize read/write to actions (the “how”, or operations)
• A protection system dictates whether a given action
performed by a given principal on a given object should be
allowed
ƒ e.g., you can read or write your files, but others cannot
ƒ e.g., your can read /etc/motd but you cannot write to it

ECS 150 (Operating Systems) File System (1), 29 Source: Gribble, Lazowska,
Levy, Zahorjan
 Model for representing protection

• Two different ways of thinking about it:

ƒ access control lists (ACLs)
o for each object, keep list of principals and principals’ allowed
actions
ƒ capabilities
o for each principal, keep list of objects and principal’s allowed
actions
• Both can be represented with the following matrix:

objects

/etc/passwd /home/gribble /home/guest

root rw rw rw

principals gribble r rw r
guest r capability

ACL
ECS 150 (Operating Systems) File System (1), 30 Source: Gribble, Lazowska,
Levy, Zahorjan
 ACLs vs. Capabilities
• Capabilities are easy to transfer
ƒ they are like keys: can hand them off
ƒ they make sharing easy
• ACLs are easier to manage
ƒ object-centric, easy to grant and revoke
o to revoke capability, need to keep track of principals that have it
o hard to do, given that principals can hand off capabilities
• ACLs grow large when object is heavily shared
ƒ can simplify by using “groups”
o put users in groups, put groups in ACLs
o you are all in the “VMware powerusers” group on Win2K
ƒ additional benefit
o change group membership, affects ALL objects that have this group
in its ACL

ECS 150 (Operating Systems) File System (1), 31 Source: Gribble, Lazowska,
Levy, Zahorjan
 Implementing Low Level Files

• Secondary storage device contains:

ƒ Volume directory (sometimes a root directory for a file
system)
ƒ External file descriptor for each file
ƒ The file contents
• Manages blocks
ƒ Assigns blocks to files (descriptor keeps track)
ƒ Keeps track of available blocks
• Maps to/from byte stream

ECS 150 (Operating Systems) File System (1), 32 Spring 2011 UC Davis
 Disk Organization

Boot Sector Volume Directory

Blk0 Blk1 … Blkk-1 Track 0, Cylinder 0
… Track 0, Cylinder 1
Blkk Blkk+1 Blk2k-1
…

Blk Blk … Blk Track 1, Cylinder 0

Blk Blk … Blk Track N-1, Cylinder 0

…
Blk Blk … Blk Track N-1, Cylinder M-1
ECS 150 (Operating Systems) File System (1), 33 Spring 2011 UC Davis
 Low-level File System Architecture

Block 0

b0 b1 b2 b3 … … bn-1

...

Sequential Device Randomly Accessed Device

ECS 150 (Operating Systems) File System (1), 34 Spring 2011 UC Davis
 File Descriptors

•External name
•Current state
•Sharable
•Owner
•User
•Locks
•Protection settings
•Length
•Time of creation
•Time of last modification
•Time of last access
•Reference count
•Storage device details

ECS 150 (Operating Systems) File System (1), 35 Spring 2011 UC Davis
 An open() Operation

• Locate the on-device (external) file descriptor

• Extract info needed to read/write file
• Authenticate that process can access the file
• Create an internal file descriptor in primary memory
• Create an entry in a “per process” open file status table
• Allocate resources, e.g., buffers, to support file usage

ECS 150 (Operating Systems) File System (1), 36 Spring 2011 UC Davis
 File Manager Data Structures

2 Keep the state

of the process-
file session
1 Copy info from
Open File external to the
3 Return a Process-File Descriptor open file
Session descriptor
reference to
the data
structure

External File Descriptor

ECS 150 (Operating Systems) File System (1), 37 Spring 2011 UC Davis
 Opening a UNIX File

fid = open(“fileA”, flags);

… On-Device File Descriptor
read(fid, buffer, len);

0 stdin
1 stdout
File structure
2 stderr
3 ...
inode

Open File Table

Internal File Descriptor

ECS 150 (Operating Systems) File System (1), 38 Spring 2011 UC Davis
 Block Management

• The job of selecting & assigning storage blocks to the

file
• For a fixed sized file of k blocks
ƒ File of length m requires N = ⎡m/k⎤ blocks
ƒ Byte bi is stored in block ⎣i/k⎦
• Three basic strategies:
ƒ Contiguous allocation
ƒ Linked lists
ƒ Indexed allocation

ECS 150 (Operating Systems) File System (1), 39 Spring 2011 UC Davis
 Contiguous Allocation

• Maps the N blocks into N contiguous blocks on the

secondary storage device
• Difficult to support dynamic file sizes

File descriptor
Head position 237
…
First block 785
Number of blocks 25

ECS 150 (Operating Systems) File System (1), 40 Spring 2011 UC Davis
ECS 150 (Operating Systems) File System (1), 41 Source: Stallings
 After Compaction
ECS 150 (Operating Systems) File System (1), 42 Source: Stallings
 Linked Lists or Chained Allocation

• Each block contains a header with

ƒ Number of bytes in the block
ƒ Pointer to next block
• Blocks need not be contiguous
• Files can expand and contract
• Seeks can be slow

First block
… Length Length Length
Byte 0 Byte 0 Byte 0
... ... ...
Head: 417 Byte 4095 Byte 4095 Byte 4095
...
Block 0 Block 1 Block N-1

ECS 150 (Operating Systems) File System (1), 43 Spring 2011 UC Davis
ECS 150 (Operating Systems) File System (1), 44 Source: Stallings
 After Consolidation
ECS 150 (Operating Systems) File System (1), 45 Source: Stallings
 Indexed Allocation

• Extract headers and put them in an index

• Simplify seeks
• May link indices together (for large files)

Byte 0
...
Index block
Byte 4095
…
Length Block 0

Length Byte 0
Head: 417 ...
... Byte 4095

Byte 0 Block 1
...
Length Byte 4095
Block N-1
ECS 150 (Operating Systems) File System (1), 46 Spring 2011 UC Davis
 Indexed allocation with Block portions
ECS 150 (Operating Systems) File System (1), 47 Source: Stallings
 Indexed allocation with variable length portions

ECS 150 (Operating Systems) File System (1), 48 Source: Stallings

 The original Unix file system
• Dennis Ritchie and Ken Thompson, Bell Labs, 1969
• “UNIX rose from the ashes of a multi-organizational effort in
the early 1960s to develop a dependable timesharing
operating system” – Multics
• Designed for a “workgroup” sharing a single system
• Did its job exceedingly well
ƒ Although it has been stretched in many directions and made
ugly in the process
• A wonderful study in engineering tradeoffs

ECS 150 (Operating Systems) File System (1), 49 Source: Gribble, Lazowska,
Levy, Zahorjan
 (Old) Unix disks are divided into five parts …
• Boot block
ƒ can boot the system by loading from this block
• Superblock
ƒ specifies boundaries of next 3 areas, and contains head of
freelists of inodes and file blocks
• i-node area
ƒ contains descriptors (i-nodes) for each file on the disk; all i-
nodes are the same size; head of freelist is in the superblock
• File contents area
ƒ fixed-size blocks; head of freelist is in the superblock
• Swap area
ƒ holds processes that have been swapped out of memory

ECS 150 (Operating Systems) File System (1), 50 Source: Gribble, Lazowska,
Levy, Zahorjan
 So …
• You can attach a disk to a dead system …
• Boot it up …
• Find, create, and modify files …
ƒ because the superblock is at a fixed place, and it tells you
where the i-node area and file contents area are
ƒ by convention, the second i-node is the root directory of the
volume

ECS 150 (Operating Systems) File System (1), 51 Source: Gribble, Lazowska,
Levy, Zahorjan
 i-node format

• User number
• Group number
• Protection bits
• Times (file last read, file last written, inode last written)
• File code: specifies if the i-node represents a directory, an
ordinary user file, or a “special file” (typically an I/O
device)
• Size: length of file in bytes
• Block list: locates contents of file (in the file contents
area)
ƒ more on this soon!
• Link count: number of directories referencing this i-node

ECS 150 (Operating Systems) File System (1), 52 Source: Gribble, Lazowska,
Levy, Zahorjan
 The flat (i-node) file system
• Each file is known by a number, which is the number of the
i-node
ƒ seriously – 1, 2, 3, etc.!
ƒ why is it called “flat”?
• Files are created empty, and grow when extended through
writes

ECS 150 (Operating Systems) File System (1), 53 Source: Gribble, Lazowska,
Levy, Zahorjan
 The tree (directory, hierarchical) file system

• A directory is a flat file of fixed-size entries

• Each entry consists of an i-node number and a file name

i-node number File name

152 .
18 ..
216 my_file
4 another_file
93 oh_my_god
144 a_directory

• It’s as simple as that!

ECS 150 (Operating Systems) File System (1), 54 Source: Gribble, Lazowska,
Levy, Zahorjan
The “block list” portion of the i-node (Unix Version 7)

• Points to blocks in the file contents area

• Must be able to represent very small and very large files.
• Each inode contains 13 block pointers
ƒ first 10 are “direct pointers” (pointers to 512B blocks of file
data)
ƒ then, single, double, and triple indirect pointers

…
1
…

…
10
…

11
12

…
…
…

ECS 150 (Operating Systems) File System (1), 55 Source: Gribble, Lazowska,
Levy, Zahorjan
 So …

• Only occupies 13 x 4B in the i-node

• Can get to 10 x 512B = a 5120B file directly
ƒ (10 direct pointers, blocks in the file contents area are
512B)
• Can get to 128 x 512B = an additional 65KB with a
single indirect reference
ƒ (the 11th pointer in the i-node gets you to a 512B block in
the file contents area that contains 128 4B pointers to
blocks holding file data)
• Can get to 128 x 128 x 512B = an additional 8MB with
a double indirect reference
ƒ (the 12th pointer in the i-node gets you to a 512B block in
the file contents area that contains 128 4B pointers to
512B blocks in the file contents area that contain 128 4B
pointers to 512B blocks holding file data)

ECS 150 (Operating Systems) File System (1), 56 Source: Gribble, Lazowska,
Levy, Zahorjan
 • Can get to 128 x 128 x 128 x 512B = an additional 1GB
with a triple indirect reference
ƒ (the 13th pointer in the i-node gets you to a 512B block in
the file contents area that contains 128 4B pointers to
512B blocks in the file contents area that contain 128 4B
pointers to 512B blocks in the file contents area that
contain 128 4B pointers to 512B blocks holding file data)
• Maximum file size is 1GB + a smidge
• Berkeley Unix went to 1KB block sizes
ƒ What’s the effect on the maximum file size?
o 256x256x256x1K = 17 GB + a smidge
ƒ What’s the price?
• Subsequently went to 4KB blocks
ƒ 1Kx1Kx1Kx4K = 4TB + a smidge

ECS 150 (Operating Systems) File System (1), 57 Source: Gribble, Lazowska,
Levy, Zahorjan
 Putting it all together

• The file system is just a huge data structure

superblock directory ‘/’

inode for ‘/’ (table of entries)

•••
directory ‘var/’
directory ‘usr/’
(table of entries)
inode for (table of entries)
inode for
‘usr/’ ‘var/’

inode for •••

‘bigfile.bin’ •••

data blocks
indirection
block
inode file block indirection block
free list free list
•••
data blocks

Indirection
block

ECS 150 (Operating Systems) File System (1), 58 Source: Gribble, Lazowska,
Levy, Zahorjan
 The UNIX File System

The steps in looking up /usr/ast/mbox

59
 File system layout
• One important goal of a file system is to lay this data
structure out on disk
ƒ have to keep in mind the physical characteristics of the disk
itself (seeks are expensive)
ƒ and the characteristics of the workload (locality across files
within a directory, sequential access to many files)
• Old UNIX’s layout is very inefficient
ƒ constantly seeking back and forth between inode area and data
block area as you traverse the file system, or even as you
sequentially read files
• Newer file systems are smarter
• Newer storage devices (SSDs) change the constraints, but
not the basic data structure

ECS 150 (Operating Systems) File System (1), 60 Source: Gribble, Lazowska,
Levy, Zahorjan
 File system consistency
• Both i-nodes and file blocks are cached in memory
• The “sync” command forces memory-resident disk
information to be written to disk
ƒ system does a sync every few seconds
• A crash or power failure between sync’s can leave an
inconsistent disk
• You could reduce the frequency of problems by reducing
caching, but performance would suffer big-time

ECS 150 (Operating Systems) File System (1), 61 Source: Gribble, Lazowska,
Levy, Zahorjan
 What do you do after a crash?
• Run a program called “fsck” to try to fix any consistency
problems
• fsck has to scan the entire disk
ƒ as disks are getting bigger, fsck is taking longer and longer
ƒ modern disks: fsck can take a full day!
• Newer file systems try to help here
ƒ are more clever about the order in which writes happen, and
where writes are directed
o e.g., Journaling file system: collect recent writes in a log called a
journal. On crash, run through journal to replay against file
system.

ECS 150 (Operating Systems) File System (1), 62 Source: Gribble, Lazowska,
Levy, Zahorjan
 VFS-based File Manager

Exports OS-specific API

File System Independent

Part of File Manager

Virtual File System Switch

MS-DOS Part of ISO 9660 Part of … ext2 Part of

File Manager File Manager File Manager

ECS 150 (Operating Systems) File System (1), 63 Spring 2011 UC Davis
 Linux VFS
• Multiple interfaces build
up VFS: File access
ƒ files
ƒ dentries
ƒ inodes
ƒ superblock
ƒ quota
• VFS can do all caching &
provides utility fctns to FS
• FS provides methods to
VFS; many are optional

ECS 150 (Operating Systems) File System (1), 64 Source: P.J.Braam, CMU
 User level file access

• Typical user level types and code:

ƒ pathnames: “/myfile”
ƒ file descriptors: fd = open(“/myfile”…)
ƒ attributes in struct stat: stat(“/myfile”,
&mybuf), chmod, chown...
ƒ offsets: write, read, lseek
ƒ directory handles: DIR *dh =
opendir(“/mydir”)
ƒ directory entries: struct dirent *ent =
readdir(dh)

ECS 150 (Operating Systems) File System (1), 65 Source: P.J.Braam, CMU
 VFS

• Manages kernel level file abstractions in

one format for all file systems
• Receives system call requests from user
level (e.g. write, open, stat, link)
• Interacts with a specific file system based
on mount point traversal
• Receives requests from other parts of the
kernel, mostly from memory management

ECS 150 (Operating Systems) File System (1), 66 Source: P.J.Braam, CMU
 File system level

• Individual File Systems

ƒ responsible for managing file & directory
data
ƒ responsible for managing meta-data:
timestamps, owners, protection etc
ƒ translates data between
o particular FS data: e.g. disk data, NFS data, Coda/AFS data
o VFS data: attributes etc in standard format

ƒ e.g. nfs_getattr(….) returns attributes in VFS

format, acquires attributes in NFS format to do
so.

ECS 150 (Operating Systems) File System (1), 67 Source: P.J.Braam, CMU
 Anatomy of stat system call
sys_stat(path, buf) {
dentry = namei(path);
if ( dentry == NULL ) return -
ENOENT; Establish VFS data

inode = dentry->d_inode;
rc =inode->i_op-
Call into inode
>i_permission(inode);
layer of filesystem
if ( rc ) return -EPERM;
Call into inode
rc = inode->i_op- layer of filesystem
>i_getattr(inode, buf);
dput(dentry);
return rc;
}
ECS 150 (Operating Systems) File System (1), 68 Source: P.J.Braam, CMU
 Anatomy of fstatfs system call

sys_fstatfs(fd, buf) { /* for things

like “df” */
file = fget(fd); Translate fd to
if ( file == NULL ) return -EBADF; VFS data
structure
superb = file->f_dentry->d_inode-
>i_super; Call into
superblock layer
rc = superb->sb_op->sb_statfs(sb, buf);
of filesystem
return rc;
}

ECS 150 (Operating Systems) File System (1), 69 Source: P.J.Braam, CMU
 DOS FAT Files
File Descriptor
43 254
… 107
Disk Disk Disk
Block Block Block
File Descriptor

43 43 254
… 107
107 Disk Disk Disk
Block Block Block

254

File Access Table (FAT)

ECS 150 (Operating Systems) File System (1), 70 Spring 2011 UC Davis