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volume groups
, 
physical volumes
, and 
logical volumes
.
A volume group is a namespace given to all the physical and logical volumes on 
your system. Basically, a volume group is the highest name that encompasses your 
entire implementation of an LVM setup. Think of it as a kind of container that is able 
to contain disks. An example of this might be a volume group named 
vg-accounting
. 
This volume group would be used for a location for the accounting department to 
store their files. It will encompass the physical volumes and logical volumes that will 
be in use by these users. It's important to note that you aren't limited to just a single 
volume group; you can have several, each with their own disks and volumes.
A physical volume is a physical or virtual hard disk that is a member of a volume 
group. For example, the hypothetical 
vg-accounting
 volume group may consist of 
three 100 GB hard disks, and each would be considered a physical volume. Keep 
in mind that these disks are still referred to as physical volumes in the context of 
LVM, even when the disks are virtual. Basically, any block device that is owned by a 
volume group is a physical volume.
Finally, logical volumes are similar in concept to partitions. Logical volumes can take 
up a portion, or the whole, of a disk, but unlike standard partitions, they may also 
span multiple disks. For example, a logical volume can include three 100 GB disks 
and be configured such that you would receive a cumulative total of 300 GB. When 
mounted, users will be able to store files there just as they would a single partition 
on a standard disk. When the volume gets full, you can add an additional disk and 
then grow the partition to increase its size. Your users would see it as a single storage 
area, even though it may consist of multiple disks.
The volume group can be named anything you'd like, but I always give mine names 
that begin with 
vg-
 and end with a name detailing its purpose. As I mentioned, you 
can have multiple volume groups. Therefore, you can have 
vg-accounting
, 
vg-sales
, 
and 
vg-techsupport
 (and so on) all on the same server. Then, you assign physical 
volumes to each. For example, you can add a 500 GB disk to your server and assign 
it to 
vg-sales
. From that point on, the 
vg-sales
 volume group owns that disk. You're 
able to split up your physical volumes any way that makes sense to you. Then, you 
can create logical volumes utilizing these physical volumes, which is what your users 
will use.

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Chapter 9