All posts for the month August, 2010

How To Create LVM Using pvcreate, vgcreate, lvcreate, and lvextend Commands

Published August 26, 2010 by Siva

LVM stands for Logical Volume Manager.

With LVM, we can create logical partitions that can span across one or more physical hard drives. First, the hard drives are divided into physical volumes, then those physical volumes are combined together to create the volume group and finally the logical volumes are created from volume group.

Before we start, install the lvm2 package as shown below.

$ yum intall lvm2

To create a LVM, we need to run through the following steps.

  • Select the physical storage devices for LVM
  • Create the Volume Group from Physical Volumes
  • Create Logical Volumes from Volume Group

Select the Physical Storage Devices for LVM – Use pvcreate, pvscan, pvdisplay Commands

In this step, we need to choose the physical volumes that will be used to create the LVM. We can create the physical volumes using pvcreate command as shown below.

$ pvcreate /dev/sda6 /dev/sda7
Physical volume "/dev/sda6" successfully created
Physical volume "/dev/sda7" successfully created

As shown above two physical volumes are created – /dev/sda6 and /dev/sda7.

If the physical volumes are already created, you can view them using the pvscan command as shown below.

$ pvscan
  PV /dev/sda6                      lvm2 [1.86 GB]
  PV /dev/sda7                      lvm2 [1.86 GB]
  Total: 2 [3.72 GB] / in use: 0 [0   ] / in no VG: 2 [3.72 GB]

You can view the list of physical volumes with attributes like size, physical extent size, total physical extent size, the free space, etc., using pvdisplay command as shown below.

$ pvdisplay
--- Physical volume ---
  PV Name             /dev/sda6
  VG Name
  PV Size               1.86 GB / not usable 2.12 MB
  Allocatable           yes
  PE Size (KByte)    4096
  Total PE              476
  Free PE               456
  Allocated PE          20
  PV UUID               m67TXf-EY6w-6LuX-NNB6-kU4L-wnk8-NjjZfv 

  --- Physical volume ---
  PV Name             /dev/sda7
  VG Name
  PV Size               1.86 GB / not usable 2.12 MB
  Allocatable           yes
  PE Size (KByte)    4096
  Total PE              476
  Free PE               476
  Allocated PE          0
  PV UUID               b031x0-6rej-BcBu-bE2C-eCXG-jObu-0Boo0x

Note : PE – Physical Extents are nothing but equal-sized chunks. The default size of extent is 4MB.

Create the Volume Group – Use vgcreate, vgdisplay Commands

Volume groups are nothing but a pool of storage that consists of one or more physical volumes. Once you create the physical volume, you can create the volume group (VG) from these physical volumes (PV).

In this example, the volume group vol_grp1 is created from the two physical volumes as shown below.

$ vgcreate vol_grp1 /dev/sda6 /dev/sda7
  Volume  group "vol_grp1" successfully created

LVM processes the storage in terms of extents. We can also change the extent size (from the default size 4MB) using -s flag.

vgdisplay command lists the created volume groups.

$ vgdisplay
  --- Volume group ---
  VG Name                     vol_grp1
  System ID
  Format                        lvm2
  Metadata Areas            2
  Metadata Sequence No  1
  VG Access                   read/write
  VG Status                    resizable
  MAX LV                       0
  Cur LV                        0
  Open LV                      0
  Max PV                       0
  Cur PV                        2
  Act PV                       2
  VG Size                      3.72 GB
  PE Size                      4.00 MB
  Total PE                     952
  Alloc PE / Size             0 / 0
  Free  PE / Size            952 / 3.72 GB
  VG UUID                     Kk1ufB-rT15-bSWe-5270-KDfZ-shUX-FUYBvR

LVM Create: Create Logical Volumes – Use lvcreate, lvdisplay command

Now, everything is ready to create the logical volumes from the volume groups. lvcreate command creates the logical volume with the size of 80MB.

$ lvcreate -l 20 -n logical_vol1 vol_grp1
  Logical volume "logical_vol1" created

Use lvdisplay command as shown below, to view the available logical volumes with its attributes.

$ lvdisplay
  --- Logical volume ---
  LV Name                /dev/vol_grp1/logical_vol1
  VG Name                vol_grp1
  LV UUID                 ap8sZ2-WqE1-6401-Kupm-DbnO-2P7g-x1HwtQ
  LV Write Access      read/write
  LV Status              available
  # open                  0
  LV Size                  80.00 MB
  Current LE              20
  Segments               1
  Allocation               inherit
  Read ahead sectors  auto
  - currently set to     256
  Block device            252:0

After creating the appropriate filesystem on the logical volumes, it becomes ready to use for the storage purpose.

$ mkfs.ext3 /dev/vol_grp1/logical_vol1

LVM resize: Change the size of the logical volumes – Use lvextend Command

We can extend the size of the logical volumes after creating it by using lvextend utility as shown below. The changes the size of the logical volume from 80MB to 100MB.

$ lvextend -L100 /dev/vol_grp1/logical_vol1
  Extending logical volume logical_vol1 to 100.00 MB
  Logical volume logical_vol1 successfully resized

We can also add additional size to a specific logical volume as shown below.

$ lvextend -L+100 /dev/vol_grp1/logical_vol1
  Extending logical volume logical_vol1 to 200.00 MB
  Logical volume logical_vol1 successfully resized

To Calculate the Database Size in MYSQL

Published August 26, 2010 by Siva

MySQL command – sum up the data_length + index_length is equal to the total table size.
data_length – store the real data.
index_length – store the table index.

List out all the databases size

SELECT table_schema "Data Base Name", sum( data_length + index_length) / 1024 / 1024 
"Data Base Size in MB" FROM information_schema.TABLES GROUP BY table_schema ;