Linux LVM Command Examples
Logical Volume Management Example 02
In this example we are going to create a loopback device to act as our disk. To do this we will use the "losetup" command. Losetup is used to associate loop devices with regular files or block devices. We are using this approach so that if you are not using a virtual environment where you can add disk, you can still try out the LVM commands.
Creating image files
The first step is to create an image file that will represent our disk:
Check what space you have available to your system with the "df -h" command:
# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/vg_centos01-lv_root
6.0G 633M 5.1G 11% /
tmpfs 250M 0 250M 0% /dev/shm
/dev/sda1 485M 38M 422M 9% /boot
From the above output we can see that we have 5.1GB of space available.
Now lets create our disk:
# fallocate -l 3G disk1.img
The above will create a file approximately 3GB in size called "disk1.img". This file will be created in the "/tmp" area as this is our current directory. We can use the "ls -lh" list command to display our new files attributes.
# ls -lh disk1.img
-rw-r--r--. 1 root root 3.0G Apr 11 08:54 disk1.img
Configure loopback device
To associate our image file with a loopback device we use the command "losetup"
# losetup /dev/loop0 /tmp/disk1.img
# losetup --show /dev/loop0
/dev/loop0: [fd00]:266586 (/tmp/disk1.img)
From the above output you can see the device "dev/loop0" is now associated to the file /tmp/disk1.img
Create a Physical Volume (PV)
Next we will use the "pvcreate" command to create our Physical Volume:
# pvcreate /dev/loop0
Writing physical volume data to disk "/dev/loop0"
Physical volume "/dev/loop0" successfully created
We can use the "pvs" command to view our results from the above:
# pvs
PV VG Fmt Attr PSize PFree
/dev/loop0 lvm2 a-- 3.00g 3.00g
/dev/sda2 vg_centos01 lvm2 a-- 7.51g 0
From the above "PV" indicates the path, "VG" indicates the Volume Group our PV belongs to. PSize and PFree indicate the amount of space available for this PV.
We can also use the "pvdisplay" command for further information:
# pvdisplay
"/dev/loop0" is a new physical volume of "3.00 GiB"
--- NEW Physical volume ---
PV Name /dev/loop0
VG Name
PV Size 3.00 GiB
Allocatable NO
PE Size 0
Total PE 0
Free PE 0
Allocated PE 0
PV UUID QAQyHk-nXFh-QV1k-TAV0-WoW7-6kiS-OFxwac
Note, the first three lines after the "--- NEW Physical volume ---" relate to the name of your PV, the VG (Volume Group) the PV is associated with and the size of the PV. In this case our PV is not currently associated with a Volume Group. This is also confirmed by the "NO" in the allocatable row.
Create a Volume Group
Next we need to add our PV to a Volume Group. To do this we use the "vgcreate" command:
# vgcreate vg01 /dev/loop0
Volume group "vg01" successfully created
We have now associated the volume group "vg01" with our device "/dev/loop0". We can use the "VGS" command to check our results:
# vgs
VG #PV #LV #SN Attr VSize VFree
vg01 1 0 0 wz--n- 3.00g 3.00g
vg_centos01 1 2 0 wz--n- 7.51g 0
From the above we can see that Volume Group "vg01" has one PV associated. We can also see that there are no Logical Volumes (LV) located within the VG.
As a check we can run the "pvs" command again. This time the output shows that our PV is now associated with Volume Group "vg01".
# pvs
PV VG Fmt Attr PSize PFree
/dev/loop0 vg01 lvm2 a-- 3.00g 3.00g
/dev/sda2 vg_centos01 lvm2 a-- 7.51g 0
Create a Logical Volume
Next we will define our Logical Volume within our Volume Group. This is where we will place our filesystem. To create our Logical Volume we will use the command "lvcreate". We are initially going to create this with a size of "2GB":
# lvcreate -n lvdata01 -L 2GB vg01
Logical volume "lvdata01" created
Now we can issue the command "lvs" to display our new Logical Volume:
# lvs
LV VG Attr LSize Pool Origin Data% Move Log Copy% Convert
lvdata01 vg01 -wi-a--- 2.00g
Now if we issue the "vgs" command again, we can see that we have a Logical Volume "vg01" with one associated PV and LV. We can also see that there is around "1Gb" of free space still available:
# vgs
VG #PV #LV #SN Attr VSize VFree
vg01 1 1 0 wz--n- 3.00g 1020.00m
For a more detailed overview of our layout we can issue the "vgdisplay" command:
# vgdisplay
--- Volume group ---
VG Name vg01
System ID
Format lvm2
Metadata Areas 1
Metadata Sequence No 2
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 1
Open LV 0
Max PV 0
Cur PV 1
Act PV 1
VG Size 3.00 GiB
PE Size 4.00 MiB
Total PE 767
Alloc PE / Size 512 / 2.00 GiB
Free PE / Size 255 / 1020.00 MiB
VG UUID V8Hfck-fl1W-m8ax-vLBp-gPSV-RR2O-LvRPyb
Create a filesystem
Our next step is to create a filesystem on our Logical Volume "/dev/vg01/lvdata01". The command used for creating filesystems is "mkfs". From my test CentOS system I have the following filesystem options available: (These may vary depending on your installation and distribution)
# mkfs
mkfs mkfs.ext2 mkfs.ext4 mkfs.xfs
mkfs.cramfs mkfs.ext3 mkfs.ext4dev
We now issue the command: mkfs.ext3 /dev/vg01/lvdata01
# mkfs.ext3 /dev/vg01/lvdata01
mke2fs 1.41.12 (17-May-2010)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
131072 inodes, 524288 blocks
26214 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=536870912
16 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 20 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
Create a Mount Point
To use our newly created filesystem we need to create a mount point for our Logical Volume. To create our mount point we use the command "mkdir":
# mkdir /var/myspace
Once this directory has been created, we next need to mount our Logical Volume to this point.
To do this we issue the "mount" command: "mount /dev/vg01/lvdata01 /var/myspace"
# mount /dev/vg01/lvdata01 /var/myspace
Now we can check that our filesystem is mounted and available for use with the "df -h" command:
# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/vg_centos01-lv_root
6.0G 3.7G 2.1G 65% /
tmpfs 250M 0 250M 0% /dev/shm
/dev/sda1 485M 38M 422M 9% /boot
/dev/mapper/vg01-lvdata01
2.0G 68M 1.9G 4% /var/myspace
From the above we can see that we have 1.9G of available space within our storage area "/var/myspace"
Mount Table
Next we will need to add our details of our device into the mount table. This will ensure that our device is automatically mounted again at boot time. The location of the mount table is: "/etc/fstab"
Add the following line into "/etc/fstab":
/dev/vg01/lvdata01 /var/myspace ext3 defaults 0 0
As this is only a mock-up of a real configuration, I am going to skip this section on my test system. On a real system you would add your details.
Increasing Space in a Logical Volume
LVM is designed to make the process of adding space to a filesystem relatively easy. In our initial allocation, we created a disk with 3GB of space, however we only allocated 2GB of the available space. If we wanted to allocate more space, we would use the "lvextend" command. Although many filesystems support online resizing, for our example I am going to unmount our device first:
# umount /var/myspace
If we use the "lvextend" command, it doesn't automatically resize the filesystem unless we supply the "-r" option. Lets check our available space again:
# lvs
LV VG Attr LSize Pool Origin Data% Move Log Copy% Convert
lvdata01 vg01 -wi-a--- 2.00g
lv_root vg_centos01 -wi-ao-- 6.04g
lv_swap vg_centos01 -wi-ao-- 1.47g
# vgs
VG #PV #LV #SN Attr VSize VFree
vg01 1 1 0 wz--n- 3.00g 1020.00m
vg_centos01 1 2 0 wz--n- 7.51g 0
# lvextend /dev/vg01/lvdata01 -L2.5G
Extending logical volume lvdata01 to 2.50 GiB
Logical volume lvdata01 successfully resized
We can now see that our Logical Volume has now changed to 2.5GB and our free space within the Volume Group "vg01" has now changed to reflect this change:
# vgs
VG #PV #LV #SN Attr VSize VFree
vg01 1 1 0 wz--n- 3.00g 508.00m
vg_centos01 1 2 0 wz--n- 7.51g 0
As we mentioned earlier, extending the Logical Volume does not automatically extend the filesystem unless the "-r" option is applied. If you do not use the "-r" option, then you will need to use the "resize2fs" command. However, we will need to run the "e2fsck" command first as a check:
# e2fsck -f /dev/vg01/lvdata01
e2fsck 1.41.12 (17-May-2010)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/vg01/lvdata01: 11/131072 files (0.0% non-contiguous), 25405/524288 blocks
Now we can run the "resize2fs" command:
# resize2fs /dev/vg01/lvdata01
resize2fs 1.41.12 (17-May-2010)
Resizing the filesystem on /dev/vg01/lvdata01 to 655360 (4k) blocks.
The filesystem on /dev/vg01/lvdata01 is now 655360 blocks long.
Next we need to mount our device again:
[root@centos var]# mount /dev/vg01/lvdata01 /var/myspace
[root@centos var]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/vg_centos01-lv_root
6.0G 3.7G 2.1G 65% /
tmpfs 250M 0 250M 0% /dev/shm
/dev/sda1 485M 38M 422M 9% /boot
/dev/mapper/vg01-lvdata01
2.5G 68M 2.3G 3% /var/myspace
Reducing amount of Space
To reduce the amount of space available, we need to use the "lvreduce" command. As a precaution, we will unmount our device first:
# umount /var/myspace
# lvreduce /dev/vg01/lvdata01 -r -L 2G
fsck from util-linux-ng 2.17.2
/dev/mapper/vg01-lvdata01: 11/163840 files (0.0% non-contiguous), 27461/655360 blocks
resize2fs 1.41.12 (17-May-2010)
Resizing the filesystem on /dev/mapper/vg01-lvdata01 to 524288 (4k) blocks.
The filesystem on /dev/mapper/vg01-lvdata01 is now 524288 blocks long.
Reducing logical volume lvdata01 to 2.00 GiB
Logical volume lvdata01 successfully resized
As you can see from the above output, we have used the "-r" option with the command:
lvreduce /dev/vg01/lvdata01 -r -L 2G
This takes care of our resizing for us by reducing the filesystem and then the Logical Volume.
Now if we check using the "lvs" and "vgs" commands we can see our changes.
# lvs
vgs LV VG Attr LSize Pool Origin Data% Move Log Copy% Convert
lvdata01 vg01 -wi-a--- 2.00g
lv_root vg_centos01 -wi-ao-- 6.04g
lv_swap vg_centos01 -wi-ao-- 1.47g
# vgs
VG #PV #LV #SN Attr VSize VFree
vg01 1 1 0 wz--n- 3.00g 1020.00m
vg_centos01 1 2 0 wz--n- 7.51g 0
Adding Another Disk
One of the most frequent tasks an administrator will be asked is to add extra space. To continue with our example, I will add an extra 1GB of disk to a loopback device "/dev/loop1".
# fallocate -l 1G /tmp/disk2.img
Now we will associate this image file with "/dev/loop1":
# losetup /dev/loop1 /tmp/disk2.img
# losetup --show /dev/loop1
/dev/loop1: [fd00]:266761 (/tmp/disk2.img)
Create a PV for the new loopback device:
# pvcreate /dev/loop1
Writing physical volume data to disk "/dev/loop1"
Physical volume "/dev/loop1" successfully created
We can issue "pvs" to display our new device:
# pvs
PV VG Fmt Attr PSize PFree
/dev/loop0 vg01 lvm2 a-- 3.00g 1020.00m
/dev/loop1 lvm2 a-- 1.00g 1.00g
/dev/sda2 vg_centos01 lvm2 a-- 7.51g 0
Now we need to add this device to our existing Volume Group "vg01":
# vgextend vg01 /dev/loop1
Volume group "vg01" successfully extended
# vgs
VG #PV #LV #SN Attr VSize VFree
vg01 2 1 0 wz--n- 3.99g 1.99g
vg_centos01 1 2 0 wz--n- 7.51g 0
We can now see that we have two Physical Volumes associated now with the Volume Group "vg01".
Now lets extend our Logical Volume to use some of this space. Again we use the "lvextend" command with the "-r" option:
# lvextend /dev/vg01/lvdata01 -r -L 3.5G
fsck from util-linux-ng 2.17.2
/dev/mapper/vg01-lvdata01: clean, 11/131072 files, 25405/524288 blocks
Extending logical volume lvdata01 to 3.50 GiB
Logical volume lvdata01 successfully resized
resize2fs 1.41.12 (17-May-2010)
Resizing the filesystem on /dev/mapper/vg01-lvdata01 to 917504 (4k) blocks.
The filesystem on /dev/mapper/vg01-lvdata01 is now 917504 blocks long.
Now we mount our device again:
# mount /dev/vg01/lvdata01 /var/myspace
# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/vg_centos01-lv_root
6.0G 4.7G 1.1G 82% /
tmpfs 250M 0 250M 0% /dev/shm
/dev/sda1 485M 38M 422M 9% /boot
/dev/mapper/vg01-lvdata01
3.5G 69M 3.3G 3% /var/myspace
We can now see the extra space is available and ready for use. Now if we issue the "pvs" command and "vgs" command we can see our changes are now reflected:
# pvs
PV VG Fmt Attr PSize PFree
/dev/loop0 vg01 lvm2 a-- 3.00g 0
/dev/loop1 vg01 lvm2 a-- 1020.00m 504.00m
/dev/sda2 vg_centos01 lvm2 a-- 7.51g 0
# vgs
VG #PV #LV #SN Attr VSize VFree
vg01 2 1 0 wz--n- 3.99g 504.00m
vg_centos01 1 2 0 wz--n- 7.51g 0
