What do first column in /proc/mounts really mean in Linux?

Question

Run cat /proc/mounts and I get this (simplified):

rootfs / rootfs rw 0 0
proc /proc proc rw,relatime 0 0
/dev/sda1 / ext3 rw,relatime,errors=continue,barrier=1,data=ordered 0 0
io /etc/blkio cgroup rw,relatime,blkio 0 0
...

About the content, document from Red Hat says that:

The first column specifies the device that is mounted, the second column reveals the mount point, and the third column tells the file system type...

I don't think that rootfs is a device, I'm trying to find this device in udev(/dev) but I can't get it. So I don't know which device is mounted in / (You may say that I can use mount to get this information, but what if rootfs here is mounted not by mount command).

Another example is on VMware ESX 3.x server, you cannot find any "vmfs" entry listed in mount's result, but by cat /proc/mounts, I get /vmfs /vmfs vmfs rw 0 0. As the rootfs example, I cannot know which devices are related with the first "/vmfs" here either.

Question: Could someone tell me what does the first column of /proc/mounts really mean?

PS. Please read "How to get the complete and exact list of mounted filesystems in Linux?" for more information about /proc/mounts and mount.

Answer 1

The first column is indeed the device as the documentation you quoted says. According to kernel.org documentation, rootfs is just a special case of a ramfs (in-RAM filesystem) that the kernel uses to make sure something is always mounted on /. It takes 0 or negligibly small space in RAM only and if you look further in /proc/mounts you should see the entry for the device containing your real root partition mounted on /.

Answer 2

The first column in /proc/mounts is the device name as recorded by the kernel. Most of the time, this is the name that was provided when calling mount (see the code of the show_vfsmnt function in the kernel); there's a way for a filesystem to tweak that name, but only NFS and AFS take advantage of this opportunity.

The name rootfs is built into the kernel; it's used for the root filesystem at boot time. There are also cases where the kernel doesn't get a name and uses none.

When you mount a filesystem that's stored on an actual device, the device name is vital: it indicates where that filesystem is. For “virtual” filesystems, the device name is often irrelevant, either there's a single possible backing (e.g. proc, sysfs, binfmt_misc) or each filesystem is independent of any backing (tmpfs) or mount options indicate where the underlying data is (most FUSE filesystems).

Answer 3

I'm not sure about rootfs but proc is a "playground" for processes. It's a virtual file system so it won't show up in /dev (I don't think) but it can be mounted.

The actual devices in /dev will be displayed in the first column with an absolute path (like you have for /dev/sda1).