Setting Up and Managing Disk Quotas - Administration

Quotas are used to limit a user’s or a group of users’ ability to consume disk space. This prevents a small group of users from monopolizing disk capacity and potentially interfering with other users or the entire system. Disk quotas are commonly used by ISPs, by Web hosting companies, on FTP sites, and on corporate file servers to ensure continued availability of their systems.

Without quotas, one or more users can upload files on an FTP server to the point of filling a filesystem. Once the affected partition is full, other users are effectively denied upload access to the disk. This is also a reason to mount different filesystem directories on different partitions. For example, if you only had partitions for your root (/) directory and swap space, someone uploading to your computer could fill up all of the space in your root directory (/). Without at least a little free space in the root directory (/), your system could become unstable or even crash.

You have two ways to set quotas for users. You can limit users by inodes or by kilobyte-sized disk blocks. Every Linux file requires an inode. Therefore, you can limit users by the number of files or by absolute space. You can set up different quotas for different filesystems. For example, you can set different quotas for users on the /home and /tmp directories if they are mounted on their own partitions.

Limits on disk blocks restrict the amount of disk space available to a user on your system. Older versions of Red Hat Linux included LinuxConf, which included a graphical tool to configure quotas. As of this writing, Red Hat no longer has a graphical quota configuration tool. Today, you can configure quotas on RHEL only through the command line interface.

ON THE JOB!  Learn to focus on command line tools. Red Hat used to make LinuxConf available as a graphical and console tool for a number of system administration functions, including quotas. While Red Hat may eventually create another GUI quota manager, don’t count on it. And GUI tools have been known to crash. On the job, as well as on the exam, command line tools are the only sure way to address just about any Linux configuration issue. Besides, command line tools are faster, and time is often of the essence on the Red Hat exams.

Quota Settings in the Kernel

By default, the Linux kernel as configured by Red Hat supports quotas. However, if you install and compile a new kernel from a remote source, you should make sure that this feature is active. The basic kernel configuration is stored in the /boot directory. For the default RHEL 3 system, you’ll find the configuration in the config-2.4.21-4.EL file. If you’ve configured a custom kernel file, you’ll find it listed under a different name.

To verify that quotas are enabled in the default RHEL 3 kernel, run the following command:

# grep CONFIG_QUOTA /boot/config-2.4.21-4.EL

There are three possible results. If you see the following, quota support is enabled:

CONFIG_QUOTA=y

Alternatively, if you see the following, quota support is not enabled:

CONFIG_QUOTA=n

If you don’t see any output, then you haven’t installed the kernel source files.

If you have a custom or upgraded kernel, use either the make menuconfig or make xconfig command to make sure support is enabled for quotas. The quota support option is located in the filesystem section. All you need to do is turn on quota support and then rebuild and install your new kernel. I’ll describe this process in more detail later in this chapter.