Red Hat ENTERPRISE LINUX 3 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual page 84

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5.5.2.2. Hierarchical Directory Structure
While the file systems used in some very old operating systems did not include the concept of direc-
tories, all commonly-used file systems today include this feature. Directories are themselves usually
implemented as files, meaning that no special utilities are required to maintain them.
Furthermore, because directories are themselves files, and directories contain files, directories can
therefore contain other directories, making a multi-level directory hierarchy possible. This is a pow-
erful concept with which all system administrators should be thoroughly familiar. Using multi-level
directory hierarchies can make file management much easer for you and for your users.
5.5.2.3. Tracking of File Creation, Access, Modification Times
Most file systems keep track of the time at which a file was created; some also track modification
and access times. Over and above the convenience of being able to determine when a given file was
created, accessed, or modified, these dates are vital for the proper operation of incremental backups.
More information on how backups make use of these file system features can be found in Section 8.2
Backups.
5.5.2.4. Access Control
Access control is one area where file systems differ dramatically. Some file systems have no clear-cut
access control model, while others are much more sophisticated. In general terms, most modern day
file systems combine two components into a cohesive access control methodology:
User identification
•
Permitted action list
•
User identification means that the file system (and the underlying operating system) must first be
capable of uniquely identifying individual users. This makes it possible to have full accountability
with respect to any operations on the file system level. Another often-helpful feature is that of user
groups — creating ad-hoc collections of users. Groups are most often used by organizations where
users may be members of one or more projects. Another feature that some file systems support is the
creation of generic identifiers that can be assigned to one or more users.
Next, the file system must be capable of maintaining lists of actions that are permitted (or not permit-
ted) against each file. The most commonly-tracked actions are:
Reading the file
•
Writing the file
•
Executing the file
•
Various file systems may extend the list to include other actions such as deleting, or even the ability
to make changes related to a file's access control.
5.5.2.5. Accounting of Space Utilized
One constant in a system administrator's life is that there is never enough free space, and even if there
is, it will not remain free for long. Therefore, a system administrator should at least be able to easily
determine the level of free space available for each file system. In addition, file systems with well-
defined user identification capabilities often include the capability to display the amount of space a
particular user has consumed.
This feature is vital in large multi-user environments, as it is an unfortunate fact of life that the 80/20
rule often applies to disk space — 20 percent of your users will be responsible for consuming 80
Chapter 5. Managing Storage