Linux – what the difference between remount to umount/mount

diskfilesystemslinuxmountrhel

When we perform this (on linux redhat 7.x)

umount /grop/sdc
umount: /grop/sdc: target is busy.
    (In some cases useful info about processes that use
     the device is found by lsof(8) or fuser(1))

We can see that mount failed on busy.

But when we do remount then …
remount is success as the following:

mount -o rw,remount /grop/sdc
echo $?
0

So very interesting.

Does remount use the option like ( umount -l ) ? what the different between remount to umount/mount ?

Best Answer

man mount :

remount

Attempt to remount an already-mounted filesystem. This is commonly used to change the mount flags for a filesystem, especially to make a readonly filesystem writeable. It does not change device or mount point. The remount functionality follows the standard way how the mount command works with options from fstab. It means the mount command doesn't read fstab (or mtab) only when a device and dir are fully specified.

The remount option is used when the file system isn't currently in use to modify the mount option from ro to rw.

target is busy.

If the file system is already in use you can't umount it properly , you need to find the process which accessed your files (fuser -mu /path/ ) , killing the running process then unmounting the file.

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