Traditionally, non-persistent names in the form of /dev/sd*(major number)**(minor number)* are used on Linux to refer to storage devices. The major and minor number range and associated sd names are allocated for each device when it is detected. This means that the association between the major and minor number range and associated sd names can change if the order of device detection changes.
Such a change in the ordering might occur in the following situations:
These reasons make it undesirable to use the major and minor number range or the associated sd names when referring to devices, such as in the /etc/fstab file.
The udev mechanism is used for all types of devices in Linux, not just for storage devices. In the case of storage devices, udev rules creates pemanent device identifiers via symbolic links in the /dev/disk/ directory. This enables you to refer to storage devices by:
đź“ť NOTE: Although udev naming attributes are persistent, in that they do not change on their own across system reboots, some are also configurable.
File system identifiers are tied to a particular file system created on a block device. The identifier is also stored as part of the file system. If you copy the file system to a different device, it still carries the same file system identifier. On the other hand, if you rewrite the device, such as by formatting it with the mkfs utility, the device loses the attribute.
File system identifiers include:
/dev/disk/by-uuid/Entries in this directory provide a symbolic name that refers to the storage device by a unique identifier (UUID). You can use the UUID to refer to the device in the /etc/fstab file using the following syntax:
UUID=3e6be9de-8139-11d1-9106-a43f08d823a6
You can configure the UUID attribute when creating a file system, and you can also change it later on.
/dev/disk/by-label/Entries in this directory provide a symbolic name that refers to the storage device by a label. You can use the label to refer to the device in the /etc/fstab file using the following syntax:
LABEL=Boot
đź“ť NOTE: Device identifiers should not be needed for the exam, but it's good knowledge to have
Device identifiers are tied to a block device: for example, a disk or a partition. If you rewrite the device, such as by formatting it with the mkfs utility, the device keeps the attribute, because it is not stored in the file system.
Device identifiers include:
Both 'UUID' and 'PARTUUID' can be used in /etc/fstab.
There are different ways to get the UUID for a partition.
Using blkid
# blkid
/dev/sda1: UUID="375fcafb-dbf0-4e72-a1de-9bba811fc6d4" BLOCK_SIZE="512" TYPE="xfs" PARTUUID="f5953a5f-01"
/dev/sda2: UUID="fv8aQX-tfJD-3ieh-G2wo-GrnR-bcML-j41Dis" TYPE="LVM2_member" PARTUUID="f5953a5f-02"
/dev/mapper/rhel-root: UUID="cc50014d-ed1f-4785-a5f2-304ec6da9002" BLOCK_SIZE="512" TYPE="xfs"
/dev/mapper/rhel-swap: UUID="01d46e11-4725-4743-8d92-ca9fdd675a8e" TYPE="swap"
Using lsblk -f
# lsblk -f
NAME FSTYPE LABEL UUID MOUNTPOINT
sda
├─sda1 xfs 375fcafb-dbf0-4e72-a1de-9bba811fc6d4 /boot
└─sda2 LVM2_member fv8aQX-tfJD-3ieh-G2wo-GrnR-bcML-j41Dis
├─rhel-root xfs cc50014d-ed1f-4785-a5f2-304ec6da9002 /
└─rhel-swap swap 01d46e11-4725-4743-8d92-ca9fdd675a8e [SWAP]
Using ls /dev/disk/by-uuid/
# ll /dev/disk/by-uuid/
total 0
lrwxrwxrwx. 1 root root 10 Nov 17 16:48 01d46e11-4725-4743-8d92-ca9fdd675a8e -> ../../dm-1
lrwxrwxrwx. 1 root root 10 Nov 17 16:48 375fcafb-dbf0-4e72-a1de-9bba811fc6d4 -> ../../sda1
lrwxrwxrwx. 1 root root 10 Nov 17 16:48 cc50014d-ed1f-4785-a5f2-304ec6da9002 -> ../../dm-0
You can also use blkid or lsblkd -f to get the UUID for logical volumes.
# pvs
PV VG Fmt Attr PSize PFree
/dev/sda2 rhel lvm2 a-- <29.00g 0
/dev/sdb vg1 lvm2 a-- <3.00g 0
/dev/sdc vg1 lvm2 a-- <3.00g 1016.00m
/dev/sdd lvm2 --- 3.00g 3.00g
/dev/sde vg1 lvm2 a-- <3.00g <3.00g
With blkid
# blkid
/dev/sda1: UUID="375fcafb-dbf0-4e72-a1de-9bba811fc6d4" BLOCK_SIZE="512" TYPE="xfs" PARTUUID="f5953a5f-01"
/dev/sda2: UUID="fv8aQX-tfJD-3ieh-G2wo-GrnR-bcML-j41Dis" TYPE="LVM2_member" PARTUUID="f5953a5f-02"
/dev/mapper/rhel-root: UUID="cc50014d-ed1f-4785-a5f2-304ec6da9002" BLOCK_SIZE="512" TYPE="xfs"
/dev/mapper/rhel-swap: UUID="01d46e11-4725-4743-8d92-ca9fdd675a8e" TYPE="swap"
/dev/sdc: UUID="qIVN0M-bpnS-2nDK-SuTE-l3bD-8Y3d-52kVwz" TYPE="LVM2_member"
/dev/sdb: UUID="8vojd0-AgE9-fDqa-pVWq-X0sl-29ll-OChdzL" TYPE="LVM2_member"
/dev/sdd: UUID="8H5qa7-7UiT-ydRL-Jc50-BSdv-weoj-4TCVJo" TYPE="LVM2_member"
/dev/sde: UUID="WwyG65-4kn1-GKHN-pgW5-X2Os-UAFc-Ca2rfB" TYPE="LVM2_member"
/dev/mapper/vg1-lv1: LABEL="lv1" UUID="83435299-3439-4db6-a7fd-ba86bbdb2387" BLOCK_SIZE="4096" TYPE="ext4"
Specifying the LV path
# blkid /dev/mapper/vg1-lv1
/dev/mapper/vg1-lv1: LABEL="lv1" UUID="83435299-3439-4db6-a7fd-ba86bbdb2387" BLOCK_SIZE="4096" TYPE="ext4"
With lsblk
# lsblk -f
NAME FSTYPE LABEL UUID MOUNTPOINT
sda
├─sda1 xfs 375fcafb-dbf0-4e72-a1de-9bba811fc6d4 /boot
└─sda2 LVM2_member fv8aQX-tfJD-3ieh-G2wo-GrnR-bcML-j41Dis
├─rhel-root xfs cc50014d-ed1f-4785-a5f2-304ec6da9002 /
└─rhel-swap swap 01d46e11-4725-4743-8d92-ca9fdd675a8e [SWAP]
sdb LVM2_member 8vojd0-AgE9-fDqa-pVWq-X0sl-29ll-OChdzL
└─vg1-lv1 ext4 lv1 83435299-3439-4db6-a7fd-ba86bbdb2387
sdc LVM2_member qIVN0M-bpnS-2nDK-SuTE-l3bD-8Y3d-52kVwz
└─vg1-lv1 ext4 lv1 83435299-3439-4db6-a7fd-ba86bbdb2387
sdd LVM2_member 8H5qa7-7UiT-ydRL-Jc50-BSdv-weoj-4TCVJo
sde LVM2_member WwyG65-4kn1-GKHN-pgW5-X2Os-UAFc-Ca2rfB
Like with UUID, you can get the label with lsblk -f
# lsblk -f
NAME FSTYPE LABEL UUID MOUNTPOINT
sda
├─sda1 xfs 375fcafb-dbf0-4e72-a1de-9bba811fc6d4 /boot
└─sda2 LVM2_member fv8aQX-tfJD-3ieh-G2wo-GrnR-bcML-j41Dis
├─rhel-root xfs cc50014d-ed1f-4785-a5f2-304ec6da9002 /
└─rhel-swap swap 01d46e11-4725-4743-8d92-ca9fdd675a8e [SWAP]
sdb LVM2_member lJvOPK-AyOu-czH5-c96Y-xsmL-2Vnn-prLgpN
└─vg1-lv1
sdc LVM2_member lAzsiO-trcw-BbGl-nbQe-bLo6-h4pr-wL09wa
└─vg1-lv1
sdd
└─sdd1 ext4 ext4fs cae6fca6-e7b4-45b4-93ea-23430424ed7a
Or with lsblk --output +LABEL
# lsblk --output +LABEL
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT LABEL
sda 8:0 0 30G 0 disk
├─sda1 8:1 0 1G 0 part /boot
└─sda2 8:2 0 29G 0 part
├─rhel-root 253:0 0 27G 0 lvm /
└─rhel-swap 253:1 0 2.1G 0 lvm [SWAP]
sdb 8:16 0 3G 0 disk
└─vg1-lv1 253:2 0 4G 0 lvm
sdc 8:32 0 3G 0 disk
└─vg1-lv1 253:2 0 4G 0 lvm
sdd 8:48 0 3G 0 disk
└─sdd1 8:49 0 3G 0 part ext4fs
With blkid
# blkid
/dev/sdd1: LABEL="ext4fs" UUID="cae6fca6-e7b4-45b4-93ea-23430424ed7a" BLOCK_SIZE="4096" TYPE="ext4" PARTLABEL="Linux filesystem" PARTUUID="a866a6e2-30be-4f5e-86e7-b5ee2830467f"
/dev/sdc: UUID="lAzsiO-trcw-BbGl-nbQe-bLo6-h4pr-wL09wa" TYPE="LVM2_member"
/dev/sdb: UUID="lJvOPK-AyOu-czH5-c96Y-xsmL-2Vnn-prLgpN" TYPE="LVM2_member"
/dev/sda1: UUID="375fcafb-dbf0-4e72-a1de-9bba811fc6d4" BLOCK_SIZE="512" TYPE="xfs" PARTUUID="f5953a5f-01"
/dev/sda2: UUID="fv8aQX-tfJD-3ieh-G2wo-GrnR-bcML-j41Dis" TYPE="LVM2_member" PARTUUID="f5953a5f-02"
/dev/mapper/rhel-root: UUID="cc50014d-ed1f-4785-a5f2-304ec6da9002" BLOCK_SIZE="512" TYPE="xfs"
/dev/mapper/rhel-swap: UUID="01d46e11-4725-4743-8d92-ca9fdd675a8e" TYPE="swap"
And with ll /dev/disk/by-label/
# ll /dev/disk/by-label/
total 0
lrwxrwxrwx. 1 root root 10 Dec 13 17:15 ext4fs -> ../../sdd1
See 5.a Create, mount, unmount, and use vfat, ext4, and xfs file systems for information on the 'fstab' and mounting filesystems.
Proceed as any other partition.
After formating the logical volume, get the UUID
# blkid /dev/mapper/vg1-lv1
/dev/mapper/vg1-lv1: LABEL="lv1" UUID="83435299-3439-4db6-a7fd-ba86bbdb2387" BLOCK_SIZE="4096" TYPE="ext4"
Create the mount point
# mkdir /mnt/lv1
Add it to /etc/fstb
# LV Mount
UUID=83435299-3439-4db6-a7fd-ba86bbdb2387 /mnt/lv1 ext4 defaults 0 2
Test the mount
# mount -a
Check that it worked
# df /mnt/lv1/
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/mapper/vg1-lv1 5095040 20472 4796040 1% /mnt/lv1
đź“ť NOTE: While you can safely use the device mapper path for LVMs, for the purposed of this objective we used the UUID.
This is good knowledge to know for the exam.
Commands:
With e2label
# e2label /dev/mapper/vg1-lv1 logical-volume1
With tune2fs
# tune2fs -L logical-volume1 /dev/mapper/vg1-lv1
Commands:
Changing the label to 'lg-vm2'
# xfs_admin -L lg-vm2 /dev/mapper/vg1-lv2
writing all SBs
new label = "lg-vm2"
# blkid /dev/mapper/vg1-lv2
/dev/mapper/vg1-lv2: LABEL="lg-vm2" UUID="145c1d64-dc00-4568-9d6a-550cd60d035d" BLOCK_SIZE="512" TYPE="xfs"