Red Hat ENTERPRISE LINUX 4 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual page 118

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5.9.7.1.4. Tracks Disk Block Usage
Disk quotas track disk block usage. Because all the data stored on a file system is stored in blocks,
disk quotas are able to directly correlate the files created and deleted on a file system with the amount
of storage those files take up.
5.9.7.1.5. Tracks Disk Inode Usage
In addition to tracking disk block usage, disk quotas also can track inode usage. Under Red Hat
Enterprise Linux, inodes are used to store various parts of the file system, but most importantly, inodes
hold information for each file. Therefore, by tracking (and controlling) inode usage, it is possible to
control the creation of new files.
5.9.7.1.6. Hard Limits
A hard limit is the absolute maximum number of disk blocks (or inodes) that can be temporarily used
by a user (or group). Any attempt to use a single block or inode above the hard limit fails.
5.9.7.1.7. Soft Limits
A soft limit is the maximum number of disk blocks (or inodes) that can be permanently used by a user
(or group).
The soft limit is set below the hard limit. This allows users to temporarily exceed their soft limit,
permitting them to finish whatever they were doing, and giving them some time in which to go through
their files and trim back their usage to below their soft limit.
5.9.7.1.8. Grace Periods
As stated earlier, any disk usage above the soft limit is temporary. It is the grace period that determines
the length of time that a user (or group) can extend their usage beyond their soft limit and toward their
hard limit.
If a user continues to use more than the soft limit and the grace period expires, no additional disk
usage will be permitted until the user (or group) has reduced their usage to a point below the soft
limit.
The grace period can be expressed in seconds, minutes, hours, days, weeks, or months, giving the
system administrator a great deal of freedom in determining how much time to give users to get their
disk usages below their soft limits.
5.9.7.2. Enabling Disk Quotas
Note
The following sections provide a brief overview of the steps necessary to enable disk quotas under
Red Hat Enterprise Linux. For a more in-depth treatment of this subject, see the chapter on disk
quotas in the Red Hat Enterprise Linux System Administration Guide.
To use disk quotas, you must first enable them. This process involves several steps:
Chapter 5. Managing Storage