Filesystem and Mount Options

In this guide, we explore how to examine and adjust filesystem mount options to optimize your system configuration. We begin with commands to inspect mounted filesystems, then move on to applying and understanding various mount options.

Previously, you may have used the lsblk command to view mounted devices briefly. While this command provides a concise overview, it lacks details like filesystem types and specific mount options. For a comprehensive view of all mounted filesystems, use the findmnt command.

For example, running findmnt might yield output similar to the following, showing that the /dev/vdb1 partition with the XFS filesystem is mounted in the mybackups directory:

$ findmnt
TARGET              SOURCE                           FSTYPE  OPTIONS
/                   /dev/mapper/ubuntu--vg-ubuntu--lv ext4    rw,relatime,seclabel
├─/proc            proc                             proc    rw,nosuid,nodev,noexec,relatime
│ ├─/proc/sys/fs/binfmt_misc binfmt_misc        binfmt_misc rw,nosuid,nodev,noexec,relatime
│ └─/proc/sys/fs/binfmt_misc binfmt_misc        binfmt_misc rw,nosuid,nodev,noexec,relatime
└─/dev             /dev/pts                         devpts  rw,nosuid,noexec,relatime,seclabel
  ├─/dev/pts      devpts                           devpts  rw,nosuid,noexec,relatime,seclabel
  ├─/dev/shm      tmpfs                            tmpfs   rw,nosuid,nodev,seclabel,inode64
  ├─/dev/mqueue   mqueue                           mqueue  rw,nosuid,nodev,noexec,relatime
  ├─/dev/hugepages hugepages                        hugepages rw,relatime,seclabel,pagesize=2M
  ├─/dev/vda2     /dev/vda2                       ext4    rw,relatime,seclabel
  └─/dev/vdb1     /dev/vdb1                       xfs     rw,relatime,seclabel,attr2,inode64
/boot              /boot                            ext4    rw,relatime,seclabel
/mybackups         /mybackups                       ext4    rw,relatime,seclabel

To filter out virtual filesystems and display only real ones (such as XFS and ext4), use the -t option:

$ findmnt -t xfs,ext4
TARGET              SOURCE                                   FSTYPE OPTIONS
/                   /dev/mapper/ubuntu--vg-ubuntu--lv          ext4 rw,relatime,seclabel
├─/boot             /dev/vda2                                  ext4 rw,relatime,seclabel
└─/mybackups        /dev/vdb1                                  xfs  rw,relatime,seclabel,attr2,inode64,logbufs=8

Note that findmnt only displays currently mounted filesystems; those defined on a partition but not mounted will not appear.

The OPTIONS column in the output indicates how each filesystem is mounted. For example, the rw option for /dev/vdb1 means that the filesystem is mounted with read-write permissions. If it were set to ro, the filesystem would be mounted as read-only, which becomes evident when you attempt to create a file:

$ findmnt -t xfs,ext4
TARGET                        SOURCE                                 FSTYPE OPTIONS
/                             /dev/mapper/ubuntu--vg-ubuntu--lv      ext4  rw,relatime,seclabel
├─/boot                      /dev/vda2                               ext4  rw,relatime,seclabel
└─/mybackups                 /dev/vdb1                               xfs   rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota

$ sudo touch /mybacksups/testfile2

Changing the mount option from rw to ro will make the filesystem read-only.

To mount a filesystem with specific options, use the -o switch with the mount command. For example, to remount a filesystem as read-only:

$ sudo touch /mybacks/testfile2
$ sudo mount -o ro /dev/vdb2 /mnt
$ findmnt -t xfs,ext4
TARGET       SOURCE                                   FSTYPE OPTIONS
/            /dev/mapper/ubuntu--vg-ubuntu--lv          ext4  rw,relatime,seclabel
├─/boot     /dev/vda2                                  ext4  rw,relatime,seclabel
└─/mybackups /dev/vdb1                                 xfs   rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota
└─/mnt      /dev/vdb2                                  ext4  ro,relatime,seclabel

$ sudo touch /mnt/testfile
touch: cannot touch '/mnt/testfile': Read-only file system

This example clearly demonstrates the effect of mount options on filesystem behavior.

Applying Multiple Mount Options

In addition to the basic ro (read-only) option, you can enhance security using options such as noexec and nosuid:

• noexec: Prevents the execution of any executable files stored on the filesystem.
• nosuid: Disables the SUID (Set User ID) bit, preventing programs from running with elevated privileges without using sudo.

These options are particularly useful on filesystems intended solely for storage (for example, on Android devices where file execution is not expected).

To mount a filesystem with these combined options, execute the following commands:

$ sudo umount /mnt
$ sudo mount -o ro,noexec,nosuid /dev/vdb2 /mnt

After mounting, verify the options with findmnt:

$ findmnt -t ext4,xfs
TARGET                          SOURCE                                     FSTYPE OPTIONS
/                               /dev/mapper/ubuntu--vg-ubuntu--lv         ext4   rw,relatime,seclabel
├─/boot                         /dev/vda2                                  ext4   rw,relatime,seclabel
├─/mybackups                   /dev/vdb1                                  xfs    rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota
└─/mnt                          /dev/vdb2                                  ext4   ro,nosuid,noexec,relatime,seclabel

If you need to modify the mount options—such as changing from read-only to read-write—you cannot simply mount again because the filesystem is already mounted. Instead, use the remount option:

$ sudo umount /mnt
$ sudo mount -o ro,noexec,nosuid /dev/vdb2 /mnt

$ findmnt -t ext4,xfs
TARGET           SOURCE                              FSTYPE OPTIONS
/                /dev/mapper/ubuntu--vg-ubuntu--lv   ext4   rw,relatime,seclabel
├─/boot         /dev/vda2                           ext4   rw,relatime,seclabel
├─/mybackups    /dev/vdb1                           xfs    rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota
└─/mnt          /dev/vdb2                           ext4   ro,nosuid,noexec,relatime,seclabel

$ sudo mount -o rw,noexec,nosuid /dev/vdb2 /mnt
mount: /mnt: /dev/sdb2 already mounted on /mnt.

$ sudo mount -o remount,rw,noexec,nosuid /dev/vdb2 /mnt

Filesystem-Specific Options

The options we have discussed so far are generally applicable across multiple filesystem types. However, some options are filesystem-specific. For instance, XFS provides an option called allocsize which defines the buffered I/O end-of-file preallocation size during delayed allocation writeout. The allocsize value can be specified in powers-of-2 increments from the page size (typically 4 KiB) up to 1 GiB. By default, XFS uses dynamic allocation based on heuristics, and specifying a fixed allocsize disables this dynamic behavior.

Before applying filesystem-dependent options, ensure that the filesystem is unmounted if necessary, then mount it with your desired option. For example:

$ sudo umount /dev/vdb1
$ sudo mount -o allocsize=32K /dev/vdb1 /mybackups

For more details on XFS-specific options, refer to the XFS manual:

$ man xfs

Automatic Mounting with /etc/fstab

So far, we have covered manual mounting techniques. These mount configurations can also be automated during system boot via the /etc/fstab file. A typical fstab entry might initially look like this:

$ sudo vim /etc/fstab
/dev/vdb1 /mybackups xfs defaults 0 2

To customize the mount options (for example, setting the filesystem to read-only with no execution), modify the entry as follows:

$ sudo vim /etc/fstab
/dev/vdb1 /mybackups xfs ro,noexec 0 2

After saving your changes and rebooting, the system will automatically apply your specified mount options.

Summary

Mount options are crucial for defining how a filesystem behaves once mounted. They control access permissions, execution capabilities, and a variety of other characteristics. By properly configuring these options, you can improve both the performance and security of your system.

This guide provides a foundation for understanding and applying various mount options. For ongoing system administration, refer to additional resources such as Mount Options Documentation and other related links.