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Chapter 5. Managing Storage

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.