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Modern Operating Systems by Herbert Bos and Andrew...
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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 345
file system marks it as unused and releases all of its blocks. This action will result
in one of the directories now pointing to an unused i-node, whose blocks may soon
be assigned to other files. Again, the solution is just to force the link count in the i-
node to the actual number of directory entries.
These two operations, checking blocks and checking directories, are often inte-
grated for efficiency reasons (i.e., only one pass over the i-nodes is required).
Other checks are also possible. For example, directories have a definite format,
with i-node numbers and ASCII names.
If an i-node number is larger than the
number of i-nodes on the disk, the directory has been damaged.
Furthermore, each i-node has a mode, some of which are legal but strange,
such as 0007, which allows the owner and his group no access at all, but allows
outsiders to read, write, and execute the file. It might be useful to at least report
files that give outsiders more rights than the owner. Directories with more than,
say, 1000 entries are also suspicious.
Files located in user directories, but which
are owned by the superuser and have the SETUID bit on, are potential security
problems because such files acquire the powers of the superuser when executed by
any user. With a little effort, one can put together a fairly long list of technically
legal but still peculiar situations that might be worth reporting.
The previous paragraphs have discussed the problem of protecting the user
against crashes. Some file systems also worry about protecting the user against
himself. If the user intends to type
to remove all the files ending with
(compiler-generated object files), but accide-
ntally types
(note the space after the asterisk),
will remove all the files in the current direc-
tory and then complain that it cannot find
In Windows, files that are removed
are placed in the recycle bin (a special directory), from which they can later be
retrieved if need be.
Of course, no storage is reclaimed until they are actually
deleted from this directory.
4.4.4 File-System Performance
Access to disk is much slower than access to memory. Reading a 32-bit memo-
ry word might take 10 nsec. Reading from a hard disk might proceed at 100
MB/sec, which is four times slower per 32-bit word, but to this must be added
5–10 msec to seek to the track and then wait for the desired sector to arrive under
the read head.
If only a single word is needed, the memory access is on the order
of a million times as fast as disk access.
As a result of this difference in access
time, many file systems have been designed with various optimizations to improve
performance. In this section we will cover three of them.

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