Extending the logical volume without unmounting the FS on Linux Servers
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root@123 ~]# lvextend -L +5G -n /dev/mapper/vg00-lv_usr
  Size of logical volume vg00/lv_usr changed from 16.00 GiB (4095 extents) to 21.00 GiB (5375 extents).
  Logical volume lv_usr successfully resized

[root@123~]# resize2fs /dev/mapper/vg00-lv_usr
resize2fs 1.41.12 (17-May-2010)
Filesystem at /dev/mapper/vg00-lv_usr is mounted on /usr; on-line resizing required
old desc_blocks = 1, new_desc_blocks = 2
Performing an on-line resize of /dev/mapper/vg00-lv_usr to 5504000 (4k) blocks.
The filesystem on /dev/mapper/vg00-lv_usr is now 5504000 blocks long.

[root@123 ~]# df -hT
Filesystem           Type   Size  Used Avail Use% Mounted on
/dev/sda2            ext3   9.7G  1.9G  7.4G  20% /
tmpfs                tmpfs  3.8G     0  3.8G   0% /dev/shm
/dev/sda1            ext3   504M  116M  363M  25% /boot
/dev/mapper/vg00-lv_home
                     ext3   2.0G  409M  1.5G  22% /home
/dev/mapper/vg00-lv_opt
                     ext3   3.0G  365M  2.5G  13% /opt
/dev/mapper/vg00-lv_service
                     ext3   5.0G  139M  4.6G   3% /service
/dev/mapper/vg00-lv_tmp
                     ext3   2.0G  301M  1.6G  16% /tmp
/dev/mapper/vg00-lv_var
                     ext3   5.0G  453M  4.3G  10% /var
/dev/mapper/vg00-lv_usr
                     ext3    21G   15G  4.7G  77% /usr
[root@123 ~]#