Managing
disk space has always been a significant task for sysadmins. Running
out of disk space used to be the start of a long and complex series of
tasks to increase the space available to a disk partition. It also
required taking the system off-line. This usually involved installing a
new hard drive, booting to recovery or single-user mode, creating a
partition and a filesystem on the new hard drive, using temporary mount
points to move the data from the too-small filesystem to the new, larger
one, changing the content of the /etc/fstab file to reflect the correct
device name for the new partition, and rebooting to remount the new
filesystem on the correct mount point.
I have to tell you that, when LVM (Logical Volume Manager) first made
its appearance in Fedora Linux, I resisted it rather strongly. My
initial reaction was that I did not need this additional layer of
abstraction between me and the hard drives. It turns out that I was
wrong, and that logical volume management is very useful.
LVM allows for very flexible disk space management. It provides
features like the ability to add disk space to a logical volume and its
filesystem while that filesystem is mounted and active and it allows for
the collection of multiple physical hard drives and partitions into a
single volume group which can then be divided into logical volumes.
The volume manager also allows reducing the amount of disk space
allocated to a logical volume, but there are a couple requirements.
First, the volume must be unmounted. Second, the filesystem itself must
be reduced in size before the volume on which it resides can be reduced.
It is important to note that the filesystem itself must allow
resizing for this feature to work. The EXT2, 3, and 4 filesystems all
allow both offline (unmounted) and online (mounted) resizing when
increasing the size of a filesystem, and offline resizing when reducing
the size. You should check the details of the filesystems you intend to
use in order to verify whether they can be resized at all and especially
whether they can be resized while online.
Expanding a filesystem on the fly
I always like to run new distributions in a VirtualBox virtual
machine for a few days or weeks to ensure that I will not run into any
devastating problems when I start installing it on my production
machines. One morning a couple years ago I started installing a newly
released version of Fedora in a virtual machine on my primary
workstation. I thought that I had enough disk space allocated to the
host filesystem in which the VM was being installed. I did not. About a
third of the way through the installation I ran out of space on that
filesystem. Fortunately, VirtualBox detected the out-of-space condition
and paused the virtual machine, and even displayed an error message
indicating the exact cause of the problem.
Note that this problem was not due to the fact that the virtual disk
was too small, it was rather the logical volume on the host computer
that was running out of space so that the virtual disk belonging to the
virtual machine did not have enough space to expand on the host's
logical volume.
Since most modern distributions use Logical Volume Management by
default, and I had some free space available on the volume group, I was
able to assign additional disk space to the appropriate logical volume
and then expand filesystem of the host on the fly. This means that I did
not have to reformat the entire hard drive and reinstall the operating
system or even reboot. I simply assigned some of the available space to
the appropriate logical volume and resized the filesystem—all while the
filesystem was on-line and the running program, The virtual machine was
still using the host filesystem. After resizing the logical volume and
the filesystem I resumed running the virtual machine and the
installation continued as if no problems had occurred.
Although this type of problem may never have happened to you, running
out of disk space while a critical program is running has happened to
many people. And while many programs, especially Windows programs, are
not as well written and resilient as VirtualBox, Linux Logical Volume
Management made it possible to recover without losing any data and
without having to restart the time-consuming installation.
LVM Structure
The structure of a Logical Volume Manager disk environment is
illustrated by Figure 1, below. Logical Volume Management enables the
combining of multiple individual hard drives and/or disk partitions into
a single volume group (VG). That volume group can then be subdivided
into logical volumes (LV) or used as a single large volume. Regular file
systems, such as EXT3 or EXT4, can then be created on a logical volume.
In Figure 1, two complete physical hard drives and one partition from
a third hard drive have been combined into a single volume group. Two
logical volumes have been created from the space in the volume group,
and a filesystem, such as an EXT3 or EXT4 filesystem has been created on
each of the two logical volumes.
Figure 1: LVM allows combining partitions and entire hard drives into Volume Groups.
Adding disk space to a host is fairly straightforward but, in my
experience, is done relatively infrequently. The basic steps needed are
listed below. You can either create an entirely new volume group or you
can add the new space to an existing volume group and either expand an
existing logical volume or create a new one.
Adding a new logical volume
There are times when it is necessary to add a new logical volume to a
host. For example, after noticing that the directory containing virtual
disks for my VirtualBox virtual machines was filling up the /home
filesystem, I decided to create a new logical volume in which to store
the virtual machine data, including the virtual disks. This would free
up a great deal of space in my /home filesystem and also allow me to
manage the disk space for the VMs independently.
The basic steps for adding a new logical volume are as follows.
- If necessary, install a new hard drive.
- Optional: Create a partition on the hard drive.
- Create a physical volume (PV) of the complete hard drive or a partition on the hard drive.
- Assign the new physical volume to an existing volume group (VG) or create a new volume group.
- Create a new logical volumes (LV) from the space in the volume group.
- Create a filesystem on the new logical volume.
- Add appropriate entries to /etc/fstab for mounting the filesystem.
- Mount the filesystem.
Now for the details. The following sequence is taken from an example I
used as a lab project when teaching about Linux filesystems.
Example
This example shows how to use the CLI to extend an existing volume
group to add more space to it, create a new logical volume in that
space, and create a filesystem on the logical volume. This procedure can
be performed on a running, mounted filesystem.
WARNING: Only the EXT3 and EXT4 filesystems can be resized on the fly
on a running, mounted filesystem. Many other filesystems including
BTRFS and ZFS cannot be resized.
Install hard drive
If there is not enough space in the volume group on the existing hard
drive(s) in the system to add the desired amount of space it may be
necessary to add a new hard drive and create the space to add to the
Logical Volume. First, install the physical hard drive, and then perform
the following steps.
Create Physical Volume from hard drive
It is first necessary to create a new Physical Volume (PV). Use the
command below, which assumes that the new hard drive is assigned as
/dev/hdd.
pvcreate /dev/hdd
It is not necessary to create a partition of any kind on the
new hard drive. This creation of the Physical Volume which will be
recognized by the Logical Volume Manager can be performed on a newly
installed raw disk or on a Linux partition of type 83. If you are going
to use the entire hard drive, creating a partition first does not offer
any particular advantages and uses disk space for metadata that could
otherwise be used as part of the PV.
Extend the existing Volume Group
In this example we will extend an existing volume group rather than
creating a new one; you can choose to do it either way. After the
Physical Volume has been created, extend the existing Volume Group (VG)
to include the space on the new PV. In this example the existing Volume
Group is named MyVG01.
vgextend /dev/MyVG01 /dev/hdd
Create the Logical Volume
First create the Logical Volume (LV) from existing free space within
the Volume Group. The command below creates a LV with a size of 50GB.
The Volume Group name is MyVG01 and the Logical Volume Name is Stuff.
lvcreate -L +50G --name Stuff MyVG01
Create the filesystem
Creating the Logical Volume does not create the filesystem. That task
must be performed separately. The command below creates an EXT4
filesystem that fits the newly created Logical Volume.
mkfs -t ext4 /dev/MyVG01/Stuff
Add a filesystem label
Adding a filesystem label makes it easy to identify the filesystem later in case of a crash or other disk related problems.
e2label /dev/MyVG01/Stuff Stuff
Mount the filesystem
At this point you can create a mount point, add an appropriate entry to the /etc/fstab file, and mount the filesystem.
You should also check to verify the volume has been created correctly. You can use the
df,
lvs, and
vgs commands to do this.
Resizing a logical volume in an LVM filesystem
The need to resize a filesystem has been around since the beginning
of the first versions of Unix and has not gone away with Linux. It has
gotten easier, however, with Logical Volume Management.
- If necessary, install a new hard drive.
- Optional: Create a partition on the hard drive.
- Create a physical volume (PV) of the complete hard drive or a partition on the hard drive.
- Assign the new physical volume to an existing volume group (VG) or create a new volume group.
- Create one or more logical volumes (LV) from the space in the volume
group, or expand an existing logical volume with some or all of the new
space in the volume group.
- If you created a new logical volume, create a filesystem on it. If
adding space to an existing logical volume, use the resize2fs command to
enlarge the filesystem to fill the space in the logical volume.
- Add appropriate entries to /etc/fstab for mounting the filesystem.
- Mount the filesystem.
Example
This example describes how to resize an existing Logical Volume in an
LVM environment using the CLI. It adds about 50GB of space to the
/Stuff filesystem. This procedure can be used on a mounted, live
filesystem only with the Linux 2.6 Kernel (and higher) and EXT3 and EXT4
filesystems. I do not recommend that you do so on any critical system,
but it can be done and I have done so many times; even on the root (/)
filesystem. Use your judgment.
WARNING: Only the EXT3 and EXT4 filesystems can be resized on the fly
on a running, mounted filesystem. Many other filesystems including
BTRFS and ZFS cannot be resized.
Install the hard drive
If there is not enough space on the existing hard drive(s) in the
system to add the desired amount of space it may be necessary to add a
new hard drive and create the space to add to the Logical Volume. First,
install the physical hard drive and then perform the following steps.
Create a Physical Volume from the hard drive
It is first necessary to create a new Physical Volume (PV). Use the
command below, which assumes that the new hard drive is assigned as
/dev/hdd.
pvcreate /dev/hdd
It is not necessary to create a partition of any kind on the
new hard drive. This creation of the Physical Volume which will be
recognized by the Logical Volume Manager can be performed on a newly
installed raw disk or on a Linux partition of type 83. If you are going
to use the entire hard drive, creating a partition first does not offer
any particular advantages and uses disk space for metadata that could
otherwise be used as part of the PV.
Add PV to existing Volume Group
For this example, we will use the new PV to extend an existing Volume
Group. After the Physical Volume has been created, extend the existing
Volume Group (VG) to include the space on the new PV. In this example,
the existing Volume Group is named MyVG01.
vgextend /dev/MyVG01 /dev/hdd
Extend the Logical Volume
Extend the Logical Volume (LV) from existing free space within the
Volume Group. The command below expands the LV by 50GB. The Volume Group
name is MyVG01 and the Logical Volume Name is Stuff.
lvextend -L +50G /dev/MyVG01/Stuff
Expand the filesystem
Extending the Logical Volume will also expand the filesystem if you
use the -r option. If you do not use the -r option, that task must be
performed separately. The command below resizes the filesystem to fit
the newly resized Logical Volume.
resize2fs /dev/MyVG01/Stuff
You should check to verify the resizing has been performed correctly. You can use the
df,
lvs, and
vgs commands to do this.
Tips
Over the years I have learned a few things that can make logical
volume management even easier than it already is. Hopefully these tips
can prove of some value to you.
- Use the Extended file systems unless you have a clear reason to use
another filesystem. Not all filesystems support resizing but EXT2, 3,
and 4 do. The EXT filesystems are also very fast and efficient. In any
event, they can be tuned by a knowledgeable sysadmin to meet the needs
of most environments if the defaults tuning parameters do not.
- Use meaningful volume and volume group names.
- Use EXT filesystem labels.
I know that, like me, many sysadmins have resisted the change to
Logical Volume Management. I hope that this article will encourage you
to at least try LVM. I am really glad that I did; my disk management
tasks are much easier since I made the switch.
A while ago, we wrote an article that described how to find out your geolocation from commandline using whereami
utility. Today, we will see how to find the geolocation of an IP
address. Of course, you can see this details from a web browser. But, it
is lot easier to find it from commandline. geoiplookup
is a command line utility that can be used to find the Country that an
IP address or hostname originates from. It uses the GeoIP library and
database to collect the details of an IP address.
