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What is UUID in fstab



What is UUID used in fstab
UUID –Universally Unique Identifier/labels
The UUID is useful for identifying hard drives in a Linux system when other factors used to locate them might change.

To find the UUIDs of all hard drives (block devices) connected to a running Linux system, open a terminal and enter sudo blkid at the command prompt. You should see something like this:
#blkid

/dev/sda1: UUID="2e2be99d-5a55-4da1-8665-219a405a8f97" TYPE="ext4"
/dev/sda5: UUID="c2f99f78-bbc9-410a-bf72-186620a8e051" TYPE="swap"
/dev/sdb1: LABEL="new" UUID="ac3ed2c8-91f1-4d31-a601-28db9a36f53a" TYPE="ext4"
/dev/sdc1: LABEL="porta" UUID="F1A5-B5DE" TYPE="vfat"
Here, we see four storage devices and their UUIDs.
  • /dev/sda1 is the system drive.
  • /dev/sda5 is the Linux swap partition.
  • /dev/sdb1 is a spare hard drive.
  • /dev/sdc1 is an external USB drive formatted as FAT32 (vfat).

 



#ls -la /dev/disk/by-uuid
 
total 0
drwxr-xr-x. 2 root root 160 Mar 25 00:24 .
drwxr-xr-x. 5 root root 100 Mar 25 00:24 ..
lrwxrwxrwx. 1 root root  10 Mar 25 00:24 0982ce66-537a-497b-baaf-99136594f3e8 -> ../../sdb1
lrwxrwxrwx. 1 root root   9 Mar 25 00:24 2014-12-02-19-30-23-00 -> ../../sr0
lrwxrwxrwx. 1 root root  10 Mar 25 00:24 5afc1b25-e6cd-45b2-ad20-69f0fed323b9 -> ../../dm-1
lrwxrwxrwx. 1 root root  10 Mar 25 00:24 8f0652a8-d79b-453f-aa2d-0ff0b5d0ae7b -> ../../dm-0
lrwxrwxrwx. 1 root root  10 Mar 25 00:24 94e15e98-1cff-49a9-b76a-a8f3a948e2ea -> ../../dm-2
lrwxrwxrwx. 1 root root  10 Mar 25 00:24 ae55a647-3c57-4ab5-9651-1389703fe6fe -> ../../sda1
The UUID is required to permanently mount specific disk drives in /etc/fstab.


dump2fs command.But the output of the command will be too long, so use pipe less to see UUID entry of the partition.

dumpe2fs /dev/sda1 | grep UUID


#tune2fs


How do I change the UUID?

This isn’t hard at all.

First find the device path

You can find the device path using the following command:
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sudo blkid
Your output will look something like this:
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sudo blkid
/dev/sdb1: UUID="aabe7e48-2d11-421f-8609-7ea9d75e7f9b" TYPE="swap"
/dev/sdc1: UUID="9467f4de-4231-401f-bcaa-fee718d49e85" TYPE="ext4"
/dev/sdc3: UUID="93a54a4a-e0f5-4152-ae59-2245e8d16ee4" TYPE="ext4"
/dev/sde5: UUID="9467f4de-4231-401f-bcaa-fee718d49e85" TYPE="ext4"
/dev/sde6: LABEL="var" UUID="30433f28-1b79-4b4d-9985-fef5b1c886b5" TYPE="ext4"
Here you can see that /dev/sdc1 and /dev/sde5 have the same UUID. The path of the partition I want to change is /dev/sde5

Secondly, generate a UUID

This is simple, the following command will output a UUID like below:
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uuidgen
f0acce91-a416-474c-8a8c-43f3ed3768f9

Finally apply the new UUID to the partition

This is also another command, tune2fs, which will apply our new UUID to our device path:
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sudo tune2fs /dev/sde5 -U f0acce91-a416-474c-8a8c-43f3ed3768f9


Linux tune2fs command examples

The tune2fs utility displays and modifies filesystem parameters on ext2,ext3,and ext4 filesystems. This utility can also setup journaling on an ext2 filesystem, turning it into ext3 filesystem.
Below is the examples show you how to use tune2fs to display, modify, and change an ext2 filesystem to ext3
Assume the filesystem has been created on /dev/md1, detail in Ext2, Ext3, and Ext4 filesystems creation on linux

Display a filesystem parameters

#tune2fs -l /dev/md1

tune2fs 1.41.12 (17-May-2010)

Filesystem volume name:   <none>

Last mounted on:          <not available>

Filesystem UUID:          957e1766-4723-420e-b942-a5dafe67c661

Filesystem magic number:  0xEF53

Filesystem revision #:    1 (dynamic)

Filesystem features:      has_journal ext_attr resize_inode dir_index filetype sparse_super large_file

Filesystem flags:         signed_directory_hash 

Default mount options:    (none)

Filesystem state:         clean

Errors behavior:          Continue

Filesystem OS type:       Linux

Inode count:              30539776

Block count:              61048976

Reserved block count:     3052448

Free blocks:              19734533

Free inodes:              30508949

First block:              0

Block size:               4096

Fragment size:            4096

Reserved GDT blocks:      1009

Blocks per group:         32768

Fragments per group:      32768

Inodes per group:         16384

Inode blocks per group:   512

Filesystem created:       Tue Sep 27 11:09:10 2011

Last mount time:          Fri Aug 21 23:28:40 2015

Last write time:          Fri Aug 21 23:33:36 2015

Mount count:              1

Maximum mount count:      32

Last checked:             Thu Aug 20 23:19:10 2015

Check interval:           15552000 (6 months)

Next check after:         Tue Feb 16 22:19:10 2016

Reserved blocks uid:      0 (user root)

Reserved blocks gid:      0 (group root)

First inode:              11

Inode size:              128

Journal inode:            8

Default directory hash:   tea

Directory Hash Seed:      0aa9310b-d299-4d55-a83b-e4eca6c0a6f2

Journal backup:           inode blocks

Modify filesystem parameters

Almost all of the parameters in previous example output can be set by tune2fs, here I jus show few of popular ones as examples, once you know how this utility works, it's easy for other options.

Set  volume label for a filesystem

The option -L volume-label is to set the volume label of the filesystem
/dev/md1 has no volume label in previous output, now set it to data
#tune2fs -L data /dev/md1

tune2fs 1.41.12 (17-May-2010)

#tune2fs -l /dev/md1 | grep volume

Filesystem volume name:   data

Change an ext2 filesystem to an ext3 filesystem

To change an ext2 filesystem to an ext3 filesystem, you must put a journal on the filesystem, and the kernel must support ext3 filesystems.
Use the option -j to set up a journal on an unmounted filesystem:
#tune2fs -j /dev/md2
Also change the entry in the fstab to reflect its new type.

To change an ext3 filesystem to an ext2 filesystem

#tune2fs -O ^has_journal /dev/md2

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