Understanding file systems and mount points as seen from df and Nautilus

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Let's say we run df command to show something about file system in my CentOS.

I can see a LVM /dev/mapper/vg_centos64-lv_root mount to /. or a partition /dev/sda1 mount to /boot, and a device CDROM /dev/sr0 mount to media/CentOS_6.5_Final.

And I can see them(filesystem) in the Nautilus. They either looks like a file or directory. Please looks below.

For my point of view. It is really confused that blending all the things(I mean the thing like partition sda1 and the things like device sr0) into the filesystem. And what the Mount doing looks like pointing a "file" to another "directory". So It really looks like that the same thing has 2 addresses to be accessed. Why does the Linux file system design like this? And Please correct me if there is something wrong with what I thought. Thanks.

I Also want to know Whether I should access from the file system or Mount point if I want? Because both are pointing to the same things.

Best Answer

In Unix everything is a file.

These files are organized in a tree structure, beginning at the root /.

Your filesystem or filesystems will then be mounted at the appropriate places in your / according your /etc/fstab file. This file contains information about your filesystems, which device they belong to and to which point they will get mounted to - the mountpoint.

Thats the "mount concept".

It is not limited to disks and other blockdevices, here are some examples involving mount:

- Mount a representation of your running kernel under /proc
- Mount a special log partition (other device, "logfriendly" filesystem) under /var/log
- Install different systems and mount just one home directory
- Mount remote directories for example via NFS to your system
- Mount a image of a cd to a specific directory

More about this topic you can find at the following url: - http://ultra.pr.erau.edu/~jaffem/tutorial/file_system_basics.htm

1. blkid

Works whether device is mounted or not.

$ blkid
/dev/sda1: UUID="XXXX" TYPE="ext4" 
/dev/sda2: UUID="XXXX" TYPE="LVM2_member"

2. mount

This is only useful once the block device has been mounted.

$ sudo mount | grep /dev/sda1
/dev/sda1 on /boot type ext4 (rw,relatime,seclabel,data=ordered)

3. lsblk

Shows drive topologies, but not the filesystem types on the devices.

$ sudo lsblk -a
NAME                      MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda                         8:0    0 465.8G  0 disk 
├─sda1                      8:1    0   500M  0 part /boot
└─sda2                      8:2    0 465.3G  0 part

tune2fs

$ tune2fs -l /dev/sda1 | grep magic
Filesystem magic number:  0xEF53

Shows the magic number associated with the device, you can look these up on this site, Linux Magic Numbers. It's also in a file, often here, /usr/share/magic. You can locate it using locate /magic.

dumpe2fs

$ sudo dumpe2fs /dev/sda1 | grep magic
dumpe2fs 1.42.7 (21-Jan-2013)
Filesystem magic number:  0xEF53

Same things apply as in tune2fs.

/dev/mounts

This is the "file" maintained by the kernel that's used to display devices that have been mounted. NOTE: many of the tools on this list are typically using the contents of this file.

$ sudo cat /proc/mounts | grep /dev/sda1
/dev/sda1 /boot ext4 rw,seclabel,relatime,data=ordered 0 0

file

You can also use the file command to display info about unmounted filesystems.

$ sudo file -s /dev/sda1
/dev/sda1: Linux rev 1.0 ext4 filesystem data, UUID=XXXX (needs journal recovery) (extents) (huge files)

An alternative syntax (as root):

$ file - </dev/sda1
/dev/stdin: Linux rev 1.0 ext4 filesystem data, UUID=XXXX (needs journal recovery) (extents) (huge files)

Your mounting issue

I'd suspect that the filesystem you're attempting to mount is not what you think it is. Either the entire device was formatted with a filesystem (/dev/sda), in which case you'd be mounting the entire drive. You can test this hypothesis like this:

$ sudo mount /dev/sda /mnt

Of if you need to explicitly tell what filesystem to use:

$ sudo mount -o ext2 /dev/sda /mnt

NOTE: This is generally not the case that an entire filesystem is laid down on the entire device, /dev/sda, rather they're partitioned into /dev/sda1, etc.

Or perhaps the drive has been partitioned. In this case you'd see these partitions with the above command lsblk, where they'd show up as /dev/sda1, /dev/sda2, etc. If that's the case for you then you'll need to mount the partition instead like so:

$ sudo mount /dev/sda1 /mnt

Again pay special attention to the formatting used on the filesystem of the device, you'll occasionally have to specify it literally to mount.

Since your output from fdisk shows that you have /dev/sda1 I'd be inclined to think that you have your filesystem on the 1st partition.

Device Boot      Start         End      Blocks   Id  System
/dev/sda1              63  1953520064   976760001   83  Linux

So mounting it like this should do the trick:

$ sudo mount -o ext2 /dev/sda1 /mnt

/etc/fstab

If the above mounting commands work then you can add this to your /etc/fstab file.

/dev/sda1 /media/HDD ext2 defaults 0 2

Linux – Trouble with understanding the concept of mounting

You can read or write /dev/cdrom (eg, using dd or cat) but when you do that you are just reading or writing the raw bytes of the device. That can be useful in various circumstances (like cloning a partition), but generally we want to see the directories and files stored on the device.

When you mount a device you're basically telling the kernel to use a layer of software (the filesystem driver) to translate those raw bytes into an actual filesystem. Thus mounting a device associates the filesystem on that device to the directory hierarchy.