edquota: Cannot stat() mounted device /dev/root

The error message "edquota: Cannot stat() mounted device /dev/root" typically indicates a problem with the quota management system on a Linux server. This problem arises when using the edquota command to edit the user quotas, provided the system cannot recognize the device file /dev/root. This error is critical to system administrators of Linux servers to understand the probable cause(s) and how the issue can be solved.

Understanding the Error

The error occurs because the system cannot locate the /dev/root device. This device is often a symbolic link to the root filesystem, which may vary depending on the system's configuration. The absence of /dev/root can lead to the inability to manage user quotas effectively, resulting in an error message when attempting to run quota-related commands.

Common Causes

1. Missing Device File: The first and most evident explanation is the absence of the /dev/root device file. This may occur when the configuration of the system is incorrect or has been modified or when the name of the device changes, especially when using different types of storage media (LVM, RAID, or cloud storage, including ISCSI).

2. Incorrect Fstab Configuration: The /etc/fstab file, which describes how types and sizes of disk partitions, filesystems, and spaces for swapping are to be mounted, may not specify user quotas. Even though the Quota Management option is installed, if the user and grp quotas are not configured for the root file system, the system will not work correctly with the user and group quotas.

3. Kernel Compatibility Issues: Occasionally, updates made to the kernel disallow correct operation with the Quota management system. For example, users have reported that after updating their kernel, they encountered issues with quotas not functioning correctly, which could also lead to the "Cannot stat" error.

4. Filesystem Type: Certain filesystem types may not support quotas or require specific configurations to enable them. For instance, the quota command will fail if the filesystem is mounted with options that do not support quotas.

Solutions

1. Verify and Create /dev/root.

First, check if the /dev/root device exists:

ls -l /dev/root

If it does not exist, then you may create it using:

mknod /dev/root b 0 0

This command creates a block device file. However, ensure you understand the implications of manually creating device files, as incorrect configurations can lead to system instability.

2. Check Fstab Configuration

Ensure that your /etc/fstab file includes the correct quota options. An example entry for the root filesystem might look like this:

/dev/sda1 / ext4 defaults, user quota, grp quota 0 1

If any alterations have been made to the/etc/fstab file, then the changes will not be immediately effective, and to see the changes, others have to remount the filesystem or reboot the server.

3. Update Kernel and Quota Tools

If the issue arose after a kernel update, consider reverting to the previous version that worked correctly. Additionally, ensure that the quota management tools are up to date. You can update your system using:

apt-get update && apt-get upgrade

Or for Red Hat-based systems:

yum update

4. Use Correct Filesystem Types

If you are using a filesystem that does not support quotas, consider switching to a compatible filesystem such as ext4 or XFS, which have robust support for user and group quotas.

5. Consult System Logs

Look for other error messages that might be stored in the system logs and that might raise more information concerning the problem. Depending on your distribution, syslogs can be found in either '/var/log/log/syslog' or '/var/log/messages' depending on your distribution.

Conclusion

The "quota: Cannot stat() mounted device /dev/root is a frequent problem in Linux systems that may result from many factors, such as the absence of the device files or improper configurations and kernel compatibility issues. By following the outlined steps, administrators can diagnose and resolve the problem effectively, ensuring user quotas are managed correctly on their systems. Regular maintenance and monitoring of filesystem configurations and kernel updates can help prevent such issues from arising in the future.