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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 327
296
FILE SYSTEMS
CHAP. 4
To make journaling work, the logged operations must be
idempotent
, which
means they can be repeated as often as necessary without harm. Operations such as
‘‘Update the bitmap to mark i-node
k
or block
n
as free’’ can be repeated until the
cows come home with no danger. Similarly, searching a directory and removing
any entry called
foobar
is also idempotent.
On the other hand, adding the newly
freed blocks from i-node
K
to the end of the free list is not idempotent since they
may already be there. The more-expensive operation ‘‘Search the list of free blocks
and add block
n
to it if it is not already present’’ is idempotent. Journaling file sys-
tems have to arrange their data structures and loggable operations so they all are
idempotent. Under these conditions, crash recovery can be made fast and secure.
For added reliability, a file system can introduce the database concept of an
atomic transaction
.
When this concept is used, a group of actions can be brack-
eted by the
begin transaction
and
end transaction
operations. The file system then
knows it must complete either all the bracketed operations or none of them, but not
any other combinations.
NTFS has an extensive journaling system and its structure is rarely corrupted
by system crashes.
It has been in development since its first release with Windows
NT in 1993.
The first Linux file system to do journaling was ReiserFS, but its pop-
ularity was impeded by the fact that it was incompatible with the then-standard
ext2 file system.
In contrast, ext3, which is a less ambitious project than ReiserFS,
also does journaling while maintaining compatibility with the previous ext2 sys-
tem.
4.3.7 Virtual File Systems
Many different file systems are in use—often on the same computer—even for
the same operating system.
A Windows system may have a main NTFS file sys-
tem, but also a legacy FAT -32 or FAT -16 drive orpartition that contains old, but
still needed, data, and from time to time a flash drive, an old CD-ROM or a DVD
(each with its own unique file system) may be required as well.
Windows handles
these disparate file systems by identifying each one with a different drive letter, as
in
C:
,
D:
, etc. When a process opens a file, the drive letter is explicitly or implicitly
present so Windows knows which file system to pass the request to. There is no at-
tempt to integrate heterogeneous file systems into a unified whole.
In contrast, all modern UNIX systems make a very serious attempt to integrate
multiple file systems into a single structure.
A Linux system could have ext2 as
the root file system, with an ext3 partition mounted on
/usr
and a second hard disk
with a ReiserFS file system mounted on
/home
as well as an ISO 9660 CD-ROM
temporarily mounted on
/mnt
.
From the user’s point of view, there is a single
file-system hierarchy. That it happens to encompass multiple (incompatible) file
systems is not visible to users or processes.
However, the presence of multiple file systems is very definitely visible to the
implementation, and since the pioneering work of Sun Microsystems (Kleiman,
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