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When examining the partition layout in Fedora 15, many experienced Linux users notice something peculiar - the root filesystem appears to be mounted twice:

[root@localhost ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
rootfs                7.9G  596M  6.9G   8% /
/dev/mapper/vg00-lv00 7.9G  596M  6.9G   8% /

rootfs is a special instance of tmpfs that the Linux kernel mounts during early boot. It serves as:

• An initial root filesystem during system initialization
• A temporary holding space before the real root filesystem is mounted
• The foundation for the Linux VFS (Virtual File System)

Here's what happens during system startup:

1. The kernel creates an empty rootfs in RAM
2. initramfs gets unpacked into rootfs
3. The real root device (/dev/mapper/vg00-lv00) is mounted
4. rootfs gets pivot_root'ed to the real root

This explains why we see both mount points - the kernel's initial rootfs and the actual root partition.

The magic happens in the kernel's initialization code. Here's a simplified view of the sequence:

start_kernel()
    vfs_caches_init()
        mnt_init()
            init_rootfs()  // Registers rootfs filesystem
            init_mount_tree()  // Creates initial rootfs mount

While this might seem confusing, it has important benefits:

• Early userspace processes can run before disk access is available
• Provides a clean way to transition to the real root filesystem
• Maintains consistency with the Linux VFS architecture

You can examine rootfs details through various methods:

# Check mount information
cat /proc/mounts | grep rootfs

# View kernel messages related to rootfs
dmesg | grep -i rootfs

# Check filesystem type
stat -f -c %T /

This behavior became more visible with:

• The adoption of systemd in Fedora 15
• Changes in the initramfs handling
• Modern kernel versions' approach to early userspace

Advanced users can modify this behavior through kernel parameters:

# To force early rootfs mounting
rootfstype=rootfs

# To debug rootfs initialization
initcall_debug

When examining the partition layout in Fedora 15 (FC15), many experienced users notice something unusual in the df -h output - the / directory appears to be mounted twice: once on rootfs and again on /dev/mapper/vg00-lv00. This isn't a bug or duplicate mount, but rather a fundamental aspect of Linux's boot process.

rootfs is a special instance of tmpfs that the kernel mounts during the early boot stages. It serves as:

• A minimal filesystem for initial boot operations
• A placeholder before the real root filesystem is mounted
• An in-memory structure (similar to other tmpfs mounts)

The sequence of events explains the apparent duplication:

1. Kernel mounts an empty rootfs as part of initialization
2. initramfs gets loaded into this rootfs
3. Device-mapper assembles LVM volumes
4. Real root filesystem (/dev/mapper/vg00-lv00) gets mounted
5. rootfs becomes hidden under the real root

This design has several advantages:

# Checking mount information shows the transition:
$ mount | grep -E '(rootfs|/)'
rootfs on / type rootfs (rw)
/dev/mapper/vg00-lv00 on / type ext4 (rw,relatime,seclabel)

The rootfs entry remains visible in df output but is effectively masked by the real root filesystem. All file operations go to the actual disk-based filesystem.

Fedora 15's approach differs from previous versions by:

If you encounter problems with this setup, consider:

# Check early boot messages:
$ dmesg | grep -i rootfs
[    0.000000] RAMDISK: rootfs image found at 0x37000000
[    1.234567] Trying to unpack rootfs image as initramfs...

Remember that rootfs is not consuming additional disk space - it's just a temporary in-memory structure that facilitates the boot process before handing over to the persistent root filesystem.