Removing a Disk from a Logical Volume

Hi, let’s see how to remove a disk from a logical volume without losing data.

This is the initial schema:

Server:
CentosLab01 (Centos 6.5)

Disk to use:
Disk /dev/sdb: 1073 MB, 1073741824 bytes (DISK A)
Disk /dev/sdc: 1073 MB, 1073741824 bytes (DISK B)
Disk /dev/sdd: 1073 MB, 1073741824 bytes (DISK C)

Creating lv_test logical volume composed by A and B disk (sdb and sdc)

1. Phycal volumes
[root@CentosLab01 ~]# pvcreate /dev/sdb Physical volume "/dev/sdb" successfully created
[root@CentosLab01 ~]# pvcreate /dev/sdc Physical volume "/dev/sdc" successfully created

2. Volume Group
[root@CentosLab01 ~]# vgcreate vg01
/dev/sdb /dev/sdc Volume group "vg01" successfully created

3. Logical Volume
[root@CentosLab01 ~]# lvcreate --extents 100%FREE --name lv_test vg01

Logical volume “lv_test” created

4. Formatting lv_test
[root@CentosLab01 ~]# mkfs.ext4 /dev/mapper/vg01-lv_test

5. Mounting disk in /media/test_disk
[root@CentosLab01 ~]# mkdir /media/my_disk

\/dev/mapper/vg01-lv_test 2.0G 35M 1.9G 2% /media/my_disk

Once we created LVM of 2gb let’s write data of 1,5 GB. We will use dd

1. Convert 1.5 GB to Bytes ( 1572864000 byte )

2. Creating files..

[root@CentosLab01 ~]# dd if=/dev/zero of=/media/my_disk/noodles.log bs=1572864000 count=1

2.0G 1.5G 371M 81% /media/my_disk

 

Keep MD5sum of noodles.log that we will compare after the disk replacing.

[root@CentosLab01 my_disk]# md5sum noodles.log

eeba7b4eee6de684b86e346a3c11d4d5  /media/my_disk/noodles.log

Create also an additional file just to be sure..

[root@CentosLab01 ~]# echo "my file " >> /media/my_disk/myfile.txt
[root@CentosLab01 ~]# cat /media/my_disk/myfile.txt
my file


Now let’s assume that DISK B (/dev/sdc) is a slow disk, instead DISK A and DISK B are fast and we have a not homogeneus situation and we want replace DISK B with DISK C (/dev/sdd).

Below the required steps:

1. make physical volume of /dev/sdd

[root@CentosLab01 ~]# pvcreate /dev/sdd
Physical volume "/dev/sdd" successfully created

2. Add sdd to the volume group VG01

[root@CentosLab01 ~]# vgextend vg01 /dev/sdd
Volume group "vg01" successfully extended
Now VG01 contains 3 disks
[root@CentosLab01 ~]# vgdisplay  -v vg01
Using volume group(s) on command line
Finding volume group "vg01"
--- Volume group ---
VG Name               vg01

PV Name               /dev/sdd
PV UUID               feeRxG-BmRR-VRcp-sAao-xTQR-blXv-bSO1qu
PV Status             allocatable

 

Now the volume group has 3 disks, the LVM has been not extended and neither the filesystems, then  we still have a /media/my_disk of 2.0 GB

 

3. Let’s see the real usage of disks

[root@CentosLab01 ~]# pvs -o+pv_used
PV         VG             Fmt  Attr PSize    PFree    Used
/dev/sda2  vg_centoslab01 lvm2 a--     7.51g       0     7.51g
/dev/sdb   vg01           lvm2 a--  1020.00m       0  1020.00m
/dev/sdc   vg01           lvm2 a--  1020.00m       0  1020.00m
/dev/sdd   vg01           lvm2 a--  1020.00m 1020.00m       0


4. Move EXTENTS from DISK B (/dev/sdc) in order to have an “empty” not used disk

[root@CentosLab01 ~]# pvmove /dev/sdc
/dev/sdc: Moved: 3.9%
/dev/sdc: Moved: 100.0%


Now DISK B has 0 used extents:

[root@CentosLab01 ~]# pvs -o+pv_used
PV         VG             Fmt  Attr PSize    PFree    Used
/dev/sda2  vg_centoslab01 lvm2 a--     7.51g       0     7.51g
/dev/sdb   vg01           lvm2 a--  1020.00m       0  1020.00m
/dev/sdc   vg01           lvm2 a--  1020.00m 1020.00m       0
/dev/sdd   vg01           lvm2 a--  1020.00m       0  1020.00m


5. We are ready to reduce the VOLUME GROUP


[root@CentosLab01 ~]# vgreduce vg01 /dev/sdc
Removed "/dev/sdc" from volume group "vg01"


The partition is still of 2.0gb

[root@CentosLab01 ~]# df -h | grep my_disk
/dev/mapper/vg01-lv_test            2.0G  1.5G  371M  81% /media/my_disk

and the data is still there 🙂
[root@CentosLab01 ~]# cat /media/my_disk/myfile.txt
my file
[root@CentosLab01 ~]# md5sum /media/my_disk/noodles.log
eeba7b4eee6de684b86e346a3c11d4d5  /media/my_disk/noodles.log

launch vgdisplay -vv vg01 for see which physical volumes are members of VG01.