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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 816
SEC. 10.6
THE LINUX FILE SYSTEM
785
are cached in what is called the
dentry
cache
.
For instance, the
dentry
cache
would contain entries for
/
,
/usr
,
/usr/ast
, and the like. If multiple processes access
the same file through the same hard link (i.e., same path), their file object will
point to the same entry in this cache.
Finally, the
file
data structure is an in-memory representation of an open file,
and is created in response to the
open
system call. It supports operations such as
read
,
write
,
sendfile
,
lock
, and other system calls described in the previous section.
The actual file systems implemented underneath the VFS need not use the
exact same abstractions and operations internally. They must, however, implement
file-system operations semantically equivalent to those specified with the VFS ob-
jects. The elements of the
operations
data structures for each of the four VFS ob-
jects are pointers to functions in the underlying file system.
The Linux Ext2 File System
We next describe one of the most popular on-disk file systems used in Linux:
ext2
.
The first Linux release used the MINIX 1 file system and was limited by
short file names and 64-MB file sizes. The MINIX 1 file system was eventually re-
placed by the first extended file system,
ext
, which permitted both longer file
names and larger file sizes. Due to its performance inefficiencies, ext was replaced
by its successor, ext2, which is still in widespread use.
An ext2 Linux disk partition contains a file system with the layout shown in
Fig. 10-31.
Block 0 is not used by Linux and contains code to boot the computer.
Following block 0, the disk partition is divided into groups of blocks, irrespective
of where the disk cylinder boundaries fall. Each group is organized as follows.
The first block is the
superblock
.
It contains information about the layout of
the file system, including the number of i-nodes, the number of disk blocks, and
the start of the list of free disk blocks (typically a few hundred entries).
Next
comes the group descriptor, which contains information about the location of the
bitmaps, the number of free blocks and i-nodes in the group, and the number of di-
rectories in the group. This information is important since ext2 attempts to spread
directories evenly over the disk.
Boot
Block group 0
Super–
Group
block
descriptor
Block group 1
Block
bitmap
Data
blocks
I–node
bitmap
I–nodes
Block group 2
Block group 3
Block group 4
...
Figure 10-31.
Disk layout of the Linux ext2 file system.
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