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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 318
SEC. 4.3
FILE-SYSTEM IMPLEMENTATION
287
The big advantage of this scheme over linked files using an in-memory table is that
the i-node need be in memory only when the corresponding file is open.
If each i-
node occupies
n
bytes and a maximum of
k
files may be open at once, the total
memory occupied by the array holding the i-nodes for the open files is only
kn
bytes. Only this much space need be reserved in advance.
File Attributes
Address of disk block 0
Address of disk block 1
Address of disk block 2
Address of disk block 3
Address of disk block 4
Address of disk block 5
Address of disk block 6
Address of disk block 7
Address of block of pointers
Disk block
containing
additional
disk addresses
Figure 4-13.
An example i-node.
This array is usually far smaller than the space occupied by the file table de-
scribed in the previous section. The reason is simple.
The table for holding the
linked list of all disk blocks is proportional in size to the disk itself.
If the disk has
n
blocks, the table needs
n
entries. As disks grow larger, this table grows linearly
with them.
In contrast, the i-node scheme requires an array in memory whose size
is proportional to the maximum number of files that may be open at once.
It does
not matter if the disk is 100 GB, 1000 GB, or 10,000 GB.
One problem with i-nodes is that if each one has room for a fixed number of
disk addresses, what happens when a file grows beyond this limit?
One solution is
to reserve the last disk address not for a data block, but instead for the address of a
block containing more disk-block addresses, as shown in Fig. 4-13. Even more ad-
vanced would be two or more such blocks containing disk addresses or even disk
blocks pointing to other disk blocks full of addresses.
We will come back to i-
nodes when studying UNIX in Chap. 10.
Similarly, the Windows NTFS file sys-
tem uses a similar idea, only with bigger i-nodes that can also contain small files.
Page 319
288
FILE SYSTEMS
CHAP. 4
4.3.3 Implementing Directories
Before a file can be read, it must be opened. When a file is opened, the operat-
ing system uses the path name supplied by the user to locate the directory entry on
the disk. The directory entry provides the information needed to find the disk
blocks. Depending on the system, this information may be the disk address of the
entire file (with contiguous allocation), the number of the first block (both link-
ed-list schemes), or the number of the i-node.
In all cases, the main function of the
directory system is to map the ASCII name of the file onto the information needed
to locate the data.
A closely related issue is where the attributes should be stored. Every file sys-
tem maintains various file attributes, such as each file’s owner and creation time,
and they must be stored somewhere. One obvious possibility is to store them di-
rectly in the directory entry.
Some systems do precisely that. This option is shown
in Fig. 4-14(a). In this simple design, a directory consists of a list of fixed-size en-
tries, one per file, containing a (fixed-length) file name, a structure of the file at-
tributes, and one or more disk addresses (up to some maximum) telling where the
disk blocks are.
(a)
games
mail
news
work
attributes
attributes
attributes
attributes
Data structure
containing the
attributes
(b)
games
mail
news
work
Figure 4-14.
(a) A simple directory containing fixed-size entries with the disk addresses
and attributes in the directory entry. (b) A directory in which each entry just
refers to an i-node.
For systems that use i-nodes, another possibility for storing the attributes is in
the i-nodes, rather than in the directory entries.
In that case, the directory entry can
be shorter: just a file name and an i-node number. This approach is illustrated in
Fig. 4-14(b).
As we shall see later, this method has some advantages over putting
them in the directory entry.
So far we have made the assumption that files have short, fixed-length names.
In MS-DOS files have a 1–8 character base name and an optional extension of 1–3
characters. In UNIX Version 7, file names were 1–14 characters, including any ex-
tensions. However, nearly all modern operating systems support longer, vari-
able-length file names. How can these be implemented?
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