This assumes you are familiar with LVM (Logical Volume Manager). If not, check out my article Getting Started with LVM (Logical Volume Manager).

Let's say you use the vgcreate command to add physical volumes to a volume group. In this example, the name of the volume group is "vg001".

vgcreate vg001 /dev/sdb1 /dev/sdc1

The vgdisplay command can then be used to display the volume group. Notice in this example that each PE (Physical Extents) is 4 MB, and that there are 3970 Physical Extents. From this we can conclude that an extent is a block of storage.

Doing some simple math, 4 times 3970 is 15880. 15880 divided by 1024 is 15.51, which matches the Volume Group size of 15.51 GB.

--- Volume group ---
VG Name               vg001
System ID
Format                lvm2
Metadata Areas        1
Metadata Sequence No  3
VG Access             read/write
VG Status             resizable
MAX LV                0
Cur LV                2
Open LV               2
Max PV                0
Cur PV                1
Act PV                1
VG Size               <15.51 GiB
PE Size               4.00 MiB
Total PE              3970
Alloc PE / Size       3960 / <15.47 GiB
Free  PE / Size       10 / 40.00 MiB
VG UUID               lgHh9m-9iMP-8ltU-2cxz-Y4wL-uwG0-EHiGFr

he lvcreate command can be used to create logical volumes from available space in the volume group. One option is to use the --size option, to declare the size in KB or MB or GB of the logical volume.

lvcreate --size 2.6G --name root vg001
lvcreate --size 10G  --name var  vg001
lvcreate --size 50G  --name www  vg001

Or the --extents option can be used, to declare the number or percentage of extents to use.

lvcreate --extents 1000     --name root vg001
lvcreate --extents 50%VG    --name var  vg001
lvcreate --extents 100%FREE --name www  vg001

The lvdisplay command can then be used to display each logical volume. Notice that each logical volume has Logical Extents (LE). Recall the each extent is 4 MB.

Notice the /dev/vg001/root logical volume has 666 Logical Extents. 666 times 4 is 2664. 2664 divided by 1024 is 2.60, which matches the Logical Volume size of 2.6 GB.

---Logical volume---
LV Path               /dev/vg001/root
LV Name               root
VG Name               vg001
LV UUID               EuBJ3c-Q8EU0M5ef-Qvip-KeyH-m2Sc-MGelKe
LV Creation host,time localhost.localdomain, 2013-10-11 15:40:12 -0500
LV Status             available
# open                1
LV Size               2.60 GiB
Current LE            666
Segments              1
Allocation            inherit
Read ahead sectors    auto
- currently set to    256
Block device          253:9

---Logical volume---
LV Path               /dev/vg001/var
LV Name               var
VG Name               vg001
LV UUID               EuBJ3c-Q8EU0M5ef-Qvip-KeyH-m2Sc-MGelKe
LV Creation host,time localhost.localdomain, 2013-10-11 15:40:12 -0500
LV Status             available
# open                1
LV Size               50.00 GiB
Current LE            12800
Segments              3
Allocation            inherit
Read ahead sectors    auto
- currently set to    256
Block device          253:9

---Logical volume---
LV Path               /dev/vg001/www
LV Name               www
VG Name               vg001
LV UUID               EuBJ3c-Q8EU0M5ef-Qvip-KeyH-m2Sc-MGelKe
LV Creation host,time localhost.localdomain, 2013-10-11 15:40:12 -0500
LV Status             available
# open                1
LV Size               10.00 GiB
Current LE            2560
Segments              3
Allocation            inherit
Read ahead sectors    auto
- currently set to    256
Block device          253:9