tmpfs

Remounting Linux Filesystems on the Fly: A Quick Reference Guide

By in Blog Tags: , , , , , , , , ,

One of the many advantages of Linux over other operating systems is its flexibility when it comes to filesystem management. A particularly useful capability is the ability to unmount and remount filesystems without requiring a system reboot. This can be invaluable when you need to make configuration changes or perform maintenance tasks while minimizing downtime.

Basic Unmounting and Remounting Procedure

The process of remounting a filesystem “on the fly” is quite straightforward. For example, to work with a tmpfs filesystem:

  1. First, unmount the filesystem:
    umount tmpfs
    This command tells the operating system to unmount the filesystem identified as “tmpfs” in the /etc/fstab file.
  2. Then, remount the filesystem:
    mount tmpfs
    This command will mount the filesystem again, applying any configuration changes you’ve made in the /etc/fstab file.

Potential Challenges and Solutions

When attempting to unmount a filesystem, you might encounter error messages if the filesystem is currently in use. Common scenarios that can prevent unmounting include:

  • Having an SSH session with your current directory inside the filesystem
  • Files being open and in use by running processes
  • System processes accessing the filesystem

If you receive an error indicating that the filesystem is busy, you’ll need to:

  1. Exit any directories within the filesystem
  2. Close any open files on the filesystem
  3. Stop any processes that might be using files on the filesystem

Once you’ve addressed these dependencies, you can try the unmount command again, and it should succeed.

Real-World Applications

There are several practical scenarios where you might need to remount a filesystem:

1. Changing Filesystem Parameters

After modifying filesystem parameters in /etc/fstab, such as adjusting the size of a tmpfs partition (as described in my previous post about resizing RAM disks), you’ll need to remount the filesystem to apply the changes.

2. Changing Mount Options

You might need to add or modify mount options such as:

  • noexec (prevent execution of binaries)
  • nosuid (ignore SUID/SGID bits)
  • ro or rw (read-only or read-write access)
  • noatime (don’t update access times)

3. Filesystem Maintenance

Some maintenance tasks require remounting with specific options, such as remounting a filesystem as read-only to run certain types of filesystem checks.

Advanced Remounting Options

For more complex situations, you can use the mount command with the -o remount option to change mount options without fully unmounting first:

mount -o remount,option1,option2 /mount/point

This approach is particularly useful for system directories that are difficult to fully unmount during operation.

Monitoring Filesystem Status

To verify the current mount status of your filesystems, use:

df -h

Or for more detailed information including mount options:

mount | grep filesystem_name

Conclusion

The ability to remount filesystems without a system restart is one of many features that make Linux an excellent choice for servers and systems requiring high availability. By understanding these basic commands, you can make configuration changes to your filesystems with minimal disruption to running services.

Remember that while this functionality is powerful, it should be used carefully, especially on production systems. Always ensure you understand the dependencies and potential impacts before unmounting any critical filesystem.

Resizing RAM Disk in Linux (/dev/shm): A Quick Guide

By in Blog Tags: , , , , , , , , ,

Linux includes a powerful feature that many system administrators find incredibly useful: a built-in RAM disk, also known as temporary filesystem or tmpfs. This feature allocates a portion of your system’s RAM for temporary storage, providing exceptionally fast read/write operations for applications that benefit from rapid access to temporary files.

Understanding the Default Configuration

By default, Linux allocates 50% of your system’s available RAM to the tmpfs mounted at /dev/shm. This standard allocation strikes a balance between providing ample space for applications that utilize tmpfs while leaving sufficient memory for other system processes.

Don’t worry about “wasting” memory—Linux’s memory management is efficient. If the RAM disk space isn’t being used, the system automatically reallocates that memory to other processes as needed. This dynamic memory management ensures optimal resource utilization regardless of your tmpfs size configuration.

When to Adjust the Default Size

There are several scenarios where modifying the default 50% allocation makes sense:

  1. Resource-intensive applications: Some applications, particularly databases with heavy temporary table usage, may benefit from a larger tmpfs allocation
  2. Systems with large RAM: On servers with substantial memory (e.g., 64GB+), 50% might be excessive for tmpfs needs
  3. Memory-constrained environments: In systems with limited RAM, you might want to reduce the tmpfs size to prioritize memory for critical applications
  4. Specific workloads: Certain workloads with predictable temporary storage needs might benefit from precise tmpfs sizing

How to Modify the RAM Disk Size

The process for adjusting your tmpfs size is straightforward and involves editing the /etc/fstab file.

Step 1: Check the Current Configuration

A default entry in your /etc/fstab file for the RAM disk typically looks like this:

tmpfs                   /dev/shm        tmpfs   defaults        0 0

This configuration uses the system’s default settings, which allocate 50% of available RAM to the tmpfs mount.

Step 2: Modify the Configuration

To change the allocation to a different percentage (for example, 75%), edit the /etc/fstab file and modify the entry to:

tmpfs                   /dev/shm        tmpfs   size=75%        0 0

You can also specify an absolute size instead of a percentage:

tmpfs                   /dev/shm        tmpfs   size=8G         0 0

This would set a fixed size of 8 gigabytes for the RAM disk, regardless of your total system memory.

Step 3: Apply the New Configuration

After modifying the /etc/fstab file, you have two options to apply the changes:

  1. Reboot the system (recommended for production environments to ensure all services properly recognize the new configuration)
  2. Remount the filesystem without rebooting: First, unmount the tmpfs:
    umount tmpfs
    Then remount it with the new settings:
    mount tmpfs

Verifying the Changes

To confirm that your changes have been successfully applied, you can check the current mount settings:

bash
df -h /dev/shm

This command will display the size of your RAM disk with the new allocation.

Best Practices

  • Monitor usage: After resizing, monitor the actual usage of your RAM disk to ensure you’ve allocated an appropriate amount
  • Document changes: Always document size changes in your system documentation for future reference
  • Consider workload patterns: Size your RAM disk based on observed workload patterns rather than arbitrary values
  • Leave room for growth: If sizing by absolute value rather than percentage, consider future memory needs

Conclusion

The ability to resize Linux’s built-in RAM disk provides system administrators with flexibility in optimizing memory allocation for specific workloads. Whether you need to increase the size for performance-critical temporary operations or decrease it to prioritize memory for other applications, the process is straightforward and can be accomplished with minimal downtime.

By understanding and properly configuring the tmpfs mounted at /dev/shm, you can significantly enhance the performance of applications that rely on fast temporary storage while maintaining efficient memory utilization across your system.

Copyright Michael Helm 2026.
The earliest version of this site available is from October 2003 - possibly viewable here: https://web.archive.org/web/20031228013629/http://www.ihelm.org.uk:80/

↑ Back to top