fstab Archives - Michael Helm's Technology Blog

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:

First, unmount the filesystem:
```
umount tmpfs
```
This command tells the operating system to unmount the filesystem identified as “tmpfs” in the /etc/fstab file.
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:

Exit any directories within the filesystem
Close any open files on the filesystem
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.

Following my previous post on creating a swap file, I realized it would be worthwhile to discuss another approach: adding an entirely new swap partition. While swap files are excellent for emergency situations, a dedicated swap partition often provides better performance for long-term use.

Prerequisites

Before we begin, you’ll need:

A separate hard drive that is either not currently mounted or can be safely unmounted using the standard umount command
Root/sudo access to your system

For this guide, I’ll assume the device is /dev/sdc, but your system might use a different device name (such as /dev/sdb, /dev/sda, /dev/hda1, etc.).

Step-by-Step Process

Ensure the drive is unmounted If the drive already contains a swap partition that’s in use, disable it first:
```
swapoff /dev/sdc
```
If the drive is clean and doesn’t have any swap partitions, you can skip this step.
Access the partition manager Launch the parted utility to manage the disk partitioning:
```
parted /dev/sdc
```
This will load the parted prompt, which provides an interface for manipulating disk partitions.
View existing partitions At the parted prompt, enter:
```
print
```
This will display details of any existing partitions and available free space on the disk.
Create the new swap partition Once you’ve identified how much space you want to allocate (at minimum, I recommend matching your RAM size), create the partition with:
```
mkpartfs part-type linux-swap start end
```
Replace start and end with appropriate values shown from the print command. For example, if the starting point is 1024MB and you want a 4GB swap partition, your end point would be 5120MB (1024 + 4096).
Exit parted Type quit to exit the parted utility. The new partition will have been assigned a number, such as sdc2 (or higher if other partitions already exist).
Format the new partition for swap Format the newly created partition as swap space:
```
mkswap /dev/sdc2
```
(Remember to replace sdc2 with your actual partition name)
Activate the swap partition Enable the swap partition with:
```
swapon /dev/sdc2
```

Making the Change Permanent

If you want the system to automatically use this swap partition on every boot, you need to add it to the file system table:

Edit the fstab file
```
vi /etc/fstab
```
Add the swap entry Press Insert to enter edit mode, then add this line to the bottom of the file:
```
/dev/sdc2 swap swap defaults 0 0
```
Save and exit Press Escape, then type :wq or press Shift+Z twice to save and quit.

Verifying the New Swap Space

To confirm that your new swap partition has been successfully added and is active, you can check the system’s swap status:

cat /proc/swaps

This will display all active swap spaces on your system, including your newly created partition.

Final Thoughts

Adding a dedicated swap partition is generally considered a more efficient long-term solution than using a swap file, especially for systems that regularly require swap space. The performance difference is modest on modern systems, but can be noticeable under heavy load.

Remember that while swap space is important for system stability, it’s not a replacement for adequate RAM. If your system is consistently using large amounts of swap space, consider upgrading your physical memory for better performance.

fstab

Remounting Linux Filesystems on the Fly: A Quick Reference Guide