TDDB68
Concurrent programming and
operating systems
File System Implementation
 File-System Structure
 File-System Implementation
 Directory Implementation
 Allocation Methods
 Free-Space Management
 Recovery
[SGG7/8/9] Chapter 11
Copyright Notice: The lecture notes are mainly based on Silberschatz’s, Galvin’s and Gagne’s book (“Operating System
Concepts”, 7th ed., Wiley, 2005). No part of the lecture notes may be reproduced in any form, due to the copyrights
reserved by Wiley. These lecture notes should only be used for internal teaching purposes at the Linköping University.
Christoph Kessler, IDA,
Linköpings universitet.
TDDB68, C. Kessler, IDA, Linköpings universitet.
File-System Structure
11.2
Silberschatz, Galvin and Gagne ©2005
In-Memory File System Structures
 File system resides on
File API:
secondary storage (disks)
 File system organized into layers:
 File control block (FCB) –
(at the logical FS layer)
storage structure consisting of
information about a file
filenames, directories,
attributes, access...
creat
Logical block addresses
(1D array of blocks)
(a) Creating a new file
Physical block addresses
on disk (cylinder, sector,...)
read/write block
commands
Layered File System
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11.3
Directory Implementation
Name
File1
File2
File3
…
Silberschatz, Galvin and Gagne ©2005
(b) Reading an open file
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11.4
Silberschatz, Galvin and Gagne ©2005
Allocation Methods
Directory
 An allocation method refers to
File pointer
F1
F2
F3
…
how disk blocks are allocated for files
Files
F1
F2
F3
F4
Fn
 Directories contain pointers to files
 Linear list of file names with pointer to the data blocks.

Contiguous allocation

Linked allocation

Indexed allocation
simple to program
 time-consuming to execute
 Possible ways to speed up lookups
 Caching lookups
 Sorting directory entries
 Hash table structure

TDDB68, C. Kessler, IDA, Linköpings universitet.
11.6
Silberschatz, Galvin and Gagne ©2005
TDDB68, C. Kessler, IDA, Linköpings universitet.
11.7
Silberschatz, Galvin and Gagne ©2005
Contiguous Allocation
Linked Allocation
 Each file occupies a set of
 Each file is a linked list of disk blocks:
contiguous blocks on the disk
blocks may be scattered
 Simple – need
only starting location
(block index)
and length (# of blocks)
block
=
next-pointer
 Random access
 Wasteful of space
 Simple –
(dynamic storageallocation problem)
need only starting address
 Free-space management
 Files cannot grow easily
 Works well on CD-ROM
TDDB68, C. Kessler, IDA, Linköpings universitet.
 No external fragmentation
Silberschatz, Galvin and Gagne ©2005
11.8
 No random access
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11.10
Silberschatz, Galvin and Gagne ©2005
File-Allocation Table (FAT)
Indexed Allocation
File-allocation table (FAT) – disk-space allocation used by MS-DOS and OS/2.
 Brings all pointers together into an index block
in FAT
Variant of linked allocation:
FAT resides in reserved section
at beginning of each disk volume
One entry for each disk block,
indexed by block number, points to successor.
Entry for last block in a chain has table value -1
Unused blocks have table value 0
 Finding free blocks is easy
Does not scale well to large disks
or small block sizes
TDDB68, C. Kessler, IDA, Linköpings universitet.
11.11
Silberschatz, Galvin and Gagne ©2005
Indexed Allocation (Cont.)
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11.12
Silberschatz, Galvin and Gagne ©2005
Multilevel-indexed allocation
 Direct access once index block is loaded

without external fragmentation,

but overhead of index block.
Directory
 All block pointers of a file must fit into the index block



How large should an index block be?
Small
– Limits file size
Large
– Wastes space for small files
Solution: Multi-level indexed allocation 
outer-index
index table
TDDB68, C. Kessler, IDA, Linköpings universitet.
11.13
Silberschatz, Galvin and Gagne ©2005
TDDB68, C. Kessler, IDA, Linköpings universitet.
11.14
file
Silberschatz, Galvin and Gagne ©2005
Combined Scheme: UNIX inode
Free-Space Management (1)
Block size 4 KB
-> With 12 direct block pointers kept in the inode, 48 KB can be addressed directly.
 Bit vector (of size n for n blocks) 0 1 2
n-1
bit[i] =
 Number of first free block =

…
1  block[i] free
0  block[i] occupied
(number of bits per word) * (number of 0-value words) + offset of first 1 bit
 Easy to get contiguous files
 Bit map requires extra space

 Small overhead
for small files
block size = 1 KB = 210 bytes
disk size = 68 GB ~ 236 bytes
n = 236/210 = 226 bits (or 67 MB)
 Still allows
large files
TDDB68, C. Kessler, IDA, Linköpings universitet.
Example:
 Inefficient unless entire bit vector is kept in main memory
11.15
Silberschatz, Galvin and Gagne ©2005
Free-Space Management (2)
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Silberschatz, Galvin and Gagne ©2005
11.16
Free-Space Management (3)
 Grouping
a really free block
(n-1 references to free blocks)
First ”free” block
 Linked free space list on disk



 Counting
Only need to store the pointer
to the first free block

Finding k free blocks means
reading in k blocks from disk

No waste of space
TDDB68, C. Kessler, IDA, Linköpings universitet.
11.17
Silberschatz, Galvin and Gagne ©2005
File System Consistency
…
Often, multiple subsequent blocks are allocated/freed
together
For sequences of free blocks located subsequently on disk,
keep only reference to first one and length of sequence
TDDB68, C. Kessler, IDA, Linköpings universitet.
11.18
Silberschatz, Galvin and Gagne ©2005
Memory-Mapped Files
 We know: Which blocks are used by each file and which blocks are
free
 If a partition was not cleanly unmounted (crash, power failure)
these can become inconsistent
 We can try to repair (fsck, scandisk)
 For each block
 Find which files use the block
 Check if the block is marked as free
 The block is used by 1 file xor is free – OK
 Two files use the same block – BAD: duplicate the block and
give one to each file
 The block is both used and is marked free – BAD: remove from
free list
 The block is neither free nor used – Wasted block: mark as free
TDDB68, C. Kessler, IDA, Linköpings universitet.
11.19
Silberschatz, Galvin and Gagne ©2005
 Memory-mapped file I/O
allows file I/O to be treated
as routine memory access
by mapping a disk block
to a page in memory
 A file is initially read using demand paging.
A page-sized portion of the file is read from the file system into
a physical page. Subsequent reads/writes to/from the file are
treated as ordinary memory accesses.
 Simplifies file access by treating file I/O through memory
rather than read() / write() system calls
 Also allows several processes to map the same file
allowing the pages in memory to be shared
TDDB68, C. Kessler, IDA, Linköpings universitet.
11.20
Silberschatz, Galvin and Gagne ©2005