15.1 LVM
Logical Volume Management or LVM are to make the handeling of disks and partitions more flexible and be able to have potentialy more space then is physically possible with physical disks. With LVM you can combine 1 or several disks or partitions into a Volume Group or VG. This can be split (partitioned) up into Logical Volumes or LV.
The steps are as follows:
Have or create partition or partitions we can use on a disk.
Designate the disk as a LVM
Physical VolumeorPVCreate a
Volume GrouporVGon thePVCreate
Logical Volumes,LVon theVGSet a Filesystem on the
LVMount or add in fstab
See the flow in the image below:
1. Create a Partition
For us to designate a Physical Volume or PV, we need a block device. A block device is a kind of file which represents a device of some kind, with data that can be read or written to it in blocks. Usually also with the abilities to seek forward and backward for the data. You can find them at /dev. To summarize: disks and partitions are block devices.
In our case, we will make two partitions on /dev/vdc of 500 Megabyte, using the fdisk command.
fdisk /dev/vdc
n - new partition
p - primairy partition
[enter] - select logical next number
[enter] - select logical partition start sector
+500M - 500 Megabyte in size
t - to label the partion
8e - selects LVM label
w - write partition
Do this *twice!!
Refresh kernel block device registry:
partprobe
Now that we have a partition (two partitions of 500M), we can designate this as a physical volume or PV
2. Designate the disk as a Physical Volume
We can designate a block device as a Physical Volume with the pvcreate command:
pvcreate /dev/vdc1 /dev/vdc2
Result:
[root@rhcsa ~]# pvcreate /dev/vdc1 /dev/vdc2
Physical volume "/dev/vdc1" successfully created.
Physical volume "/dev/vdc2" successfully created.
This will have labeled the device as a Physical Volume, with the standard extend size 4.00 MiB per extend.
You can check the status of the pv with pvdisplay:
pvdisplay
Result:
[root@rhcsa ~]# pvdisplay
--- Physical volume ---
PV Name /dev/vdc1
VG Name plato
PV Size 500.00 MiB / not usable 4.00 MiB
Allocatable yes (but full)
PE Size 4.00 MiB
Total PE 124
Free PE 124
Allocated PE 0
PV UUID Il1ImX-zOLu-G9gz-APwS-2SZ6-KWb0-vHE7y9
--- Physical volume ---
PV Name /dev/vdc2
VG Name plato
PV Size 500.00 MiB / not usable 4.00 MiB
Allocatable yes
PE Size 4.00 MiB
Total PE 124
Free PE 124
Allocated PE 0
PV UUID WsBS2y-x8fr-caYd-iFOA-G05q-NHwB-OM4H2y
--- Physical volume ---
PV Name /dev/vda2
VG Name cl
PV Size <8.00 GiB / not usable 3.00 MiB
Allocatable yes (but full)
PE Size 4.00 MiB
Total PE 2047
Free PE 0
Allocated PE 2047
PV UUID aq53k3-yETG-pEsz-COhh-W902-2drm-vdOJPL
3. Create a Volume Group
Next up we need to create a new Volume Group of the Physical Volume or volumes.
We will create a Volume Group named plato: this is done with the vgcreate command.
vgcreate plato /dev/vdc1 /dev/vdc2
Result:
[root@rhcsa ~]# vgcreate plato /dev/vdc1 /dev/vdc2
Volume group "plato" successfully created
This will create a Volume Group for us, on which we can now create Logical Volumes.
We can check the display status of the VG with vgdisplay.
vgdisplay plato
[root@rhcsa ~]# vgdisplay plato
--- Volume group ---
VG Name plato
System ID
Format lvm2
Metadata Areas 2
Metadata Sequence No 3
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 2
Open LV 0
Max PV 0
Cur PV 2
Act PV 2
VG Size 992.00 MiB
PE Size 4.00 MiB
Total PE 248
Alloc PE / Size 0 / 980.00 MiB
Free PE / Size 248 / 12.00 MiB
VG UUID 44avDc-4ODt-xaeZ-MGAL-tEzx-efEC-c2QWl2
4. Create Logical Volumes
We will create two Logical Volumes for us to experiment with: cave and phaedrus.
This can be done with the lvcreate command. If we want to give a new name to the volume, we use the -n switch to allow for a name input. The -L input is for how big you want the volume to be in (M or G).
lvcreate -n cave -L 500M plato
Result:
[root@rhcsa ~]# lvcreate -n cave -L 500M plato
Logical volume "cave" created.
This will create a device called /dev/plato/cave of 500M, currently without a file system on it.
Next up, we want to create phaedrus. But instead of determining the size, we want to create phaedrus by the amount of extends. 120 extends of 4MB; this should be about 480M (120 Extends * 4Mb = 480Mb). Extends are done with the small -l switch.
lvcreate -n phaedrus -l 120 plato
Result:
[root@rhcsa ~]# lvcreate -n phaedrus -l 120 plato
Logical volume "phaedrus" created.
Let’s check our results with the (you might have guessed it) lvdisplay command:
lvdisplay plato
Result:
[root@rhcsa ~]# lvdisplay plato
--- Logical volume ---
LV Path /dev/plato/cave
LV Name cave
VG Name plato
LV UUID h14a4t-kktR-Y5cp-DPT0-JDFe-GUeS-vNJVjW
LV Write Access read/write
LV Creation host, time rhcsa.greateracademy.local, 2020-08-10 11:41:15 -0400
LV Status available
# open 0
LV Size 500.00 MiB
Current LE 125
Segments 2
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 253:3
--- Logical volume ---
LV Path /dev/plato/phaedrus
LV Name phaedrus
VG Name plato
LV UUID LRrN1e-X7es-W0Bb-H22u-XZcS-vHnC-fjhl6D
LV Write Access read/write
LV Creation host, time rhcsa.greateracademy.local, 2020-08-10 11:46:29 -0400
LV Status available
# open 0
LV Size 480.00 MiB
Current LE 120
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 253:4
Now that we have our Logical Volumes, the next step is to add filesystems on them.
5. Set a Filesystem on the LV
The next part is creating a filesystem on the Logical Volume so we can start using the space for files. We will make one ext4 and another xfs.
We can do this like other excersizes with the mkfs command. We will show you 2 methods:
mkfs.xfs /dev/mapper/plato-cave
Result:
[root@rhcsa ~]# mkfs.xfs /dev/mapper/plato-cave
meta-data=/dev/mapper/plato-cave isize=512 agcount=4, agsize=32000 blks
= sectsz=512 attr=2, projid32bit=1
= crc=1 finobt=1, sparse=1, rmapbt=0
= reflink=1
data = bsize=4096 blocks=128000, imaxpct=25
= sunit=0 swidth=0 blks
naming =version 2 bsize=4096 ascii-ci=0, ftype=1
log =internal log bsize=4096 blocks=1368, version=2
= sectsz=512 sunit=0 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0
And the second example:
mkfs.ext4 /dev/plato/phaedrus
Result:
[root@rhcsa ~]# mkfs.ext4 /dev/plato/phaedrus
mke2fs 1.44.3 (10-July-2018)
Discarding device blocks: done
Creating filesystem with 491520 1k blocks and 122880 inodes
Filesystem UUID: eb3056b5-b333-4149-b070-84d3fd14aea6
Superblock backups stored on blocks:
8193, 24577, 40961, 57345, 73729, 204801, 221185, 401409
Allocating group tables: done
Writing inode tables: done
Creating journal (8192 blocks): done
Writing superblocks and filesystem accounting information: done
Now that we added filesystems to the LV, we can now persistently mount them with /etc/fstab.
6. Mount or add in fstab
To add them to the /etc/fstab, we need to add the blockID to the fstab like we did before. We will add plato-cave to /mnt/cave and plato-phaedrus to /mnt/phaedrus.
Let’s make the directories for the mounts:
mkdir -p /mnt/cave ; mkdir -p /mnt/phaedrus
Now let’s add the blockid’s to the /etc/fstab.
First we create a backup:
cp /etc/fstab /etc/fstab.bak
Next we will push the cave and the phaedrus blkid’s into the /etc/fstab:
blkid | grep cave >> /etc/fstab; blkid | grep phaedrus >> /etc/fstab
Edit the /etc/fstab with vim:
vim /etc/fstab
It should look like this:
/dev/mapper/cl-root / xfs defaults 0 0
UUID=8503adac-d001-46c0-92b5-d2c14d46c916 /boot ext4 defaults 1 2
/dev/mapper/cl-swap swap swap defaults 0 0
/dev/mapper/plato-cave: UUID="6711d453-c6cd-4b81-92c5-eafc877d9670" TYPE="xfs"
/dev/mapper/plato-phaedrus: UUID="eb3056b5-b333-4149-b070-84d3fd14aea6" TYPE="ext4"
Make the changes to make it look as following:
/dev/mapper/cl-root / xfs defaults 0 0
UUID=8503adac-d001-46c0-92b5-d2c14d46c916 /boot ext4 defaults 1 2
/dev/mapper/cl-swap swap swap defaults 0 0
UUID=6711d453-c6cd-4b81-92c5-eafc877d9670 /mnt/cave xfs defaults 0 0
UUID=eb3056b5-b333-4149-b070-84d3fd14aea6 /mnt/phaedrus ext4 defaults 0 0
We can now mount the devices using mount command:
mount -va
Result:
[root@rhcsa ~]# mount -va
/ : ignored
/boot : already mounted
swap : ignored
mount: /mnt/cave does not contain SELinux labels.
You just mounted an file system that supports labels which does not
contain labels, onto an SELinux box. It is likely that confined
applications will generate AVC messages and not be allowed access to
this file system. For more details see restorecon(8) and mount(8).
/mnt/cave : successfully mounted
mount: /mnt/phaedrus does not contain SELinux labels.
You just mounted an file system that supports labels which does not
contain labels, onto an SELinux box. It is likely that confined
applications will generate AVC messages and not be allowed access to
this file system. For more details see restorecon(8) and mount(8).
/mnt/phaedrus : successfully mounted
If you wish to fix the selinux messages from your mount command:
restorecon -RFvv /mnt/
Result:
[root@rhcsa ~]# restorecon -RFvv /mnt/
Relabeled /mnt/phaedrus from system_u:object_r:unlabeled_t:s0 to system_u:object_r:mnt_t:s0
Relabeled /mnt/cave from system_u:object_r:unlabeled_t:s0 to system_u:object_r:mnt_t:s0
and lsblk to check the results of what you made:
lsblk
Result:
[root@rhcsa ~]# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
vda 8:0 0 9G 0 disk
├─vda1 8:1 0 1G 0 part /boot
└─vda2 8:2 0 8G 0 part
├─cl-root 253:0 0 6.1G 0 lvm /
├─cl-swap 253:1 0 924M 0 lvm [SWAP]
vdb 8:16 0 10G 0 disk
vdc 8:32 0 5G 0 disk
├─vdc1 8:33 0 500M 0 part
│ └─plato-cave 253:3 0 500M 0 lvm /mnt/cave
└─vdc2 8:34 0 500M 0 part
├─plato-cave 253:3 0 500M 0 lvm /mnt/cave
└─plato-phaedrus 253:4 0 480M 0 lvm /mnt/phaedrus
sr0 11:0 1 1024M 0 rom
Now you have created, added and mounted all the devices. Well done!