LVM understanding

lvm understadning -  http://ostechnix.wordpress.com/2013/02/03/linux-basics-lvm-logical-volume-manager-tutorial/

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we use fdisk to format the raw hard disk 

fdisk /dev/sdb

/dev/sdb1               1          14      112423+  83  Linux
/dev/sdb2              15          28      112455   83  Linux
/dev/sdb3              29          42      112455   83  Linux

partprobe

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now we create a physical volume

pvcreate /dev/sdb1 /dev/sdb2 /dev/sdb3 
pvdisplay

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We create the volume group vg1

vgcreate vg1 /dev/sdb1 /dev/sdb2 
vgdisplay

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We now create the logical volume

lvcreate -L 200M vg1 -n lv1
lvdisplay 
mkfs.ext4 /dev/vg1/lv1
mount    /dev/vg1/lv1   /mnt/

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to extend new hdd :

pvcreate  /dev/sdb1
vgextend vg1 /dev/sdb1
lvresize -L +100M /dev/vg1/lv1
resize2fs /dev/vg1/lv1
lvdisplay /dev/vg1/lv1

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the following procedure, in this example we shrink a partition from 10G to 9G:

First, we unmount.

# umount /dev/vg_blah/lv_blah

Check the file system.

# e2fsck -f /dev/vg_blah/lv_blah

Resize the file system.

# resize2fs -p /dev/vg_blah/lv_blah 9G

Reduce the size of the logical volume.

# lvreduce -L -1G /dev/vg_blah/lv_blah

Shrink the volume group if desired.

# vgreduce vg_blah /dev/sdxy