Linux Tips

Managing Physical and Logical Volumes

by Jeff Hunter, Sr. Database Administrator

Contents

• Introduction
• Create Logical Volumes
• Delete Logical Volumes
• About the Author

Introduction

The following article presents an overview of the commands used to manage physical and logical volumes for use with Logical Volume Manager (LVM) in Linux.

Before considering the various commands for LVM, lets first look at just what is meant by some of the terminology of LVM.

A logical volume lives in a volume group that is made up of one or more physical volumes. All volume groups are part of the Logical Volume Manager. Here is a table that lists the three types of Logical Volume's.

Logical Volume Types

Logical Volume	Physical Device	File System
volume group	one or more disks	vgdisplay
physical volume group	physical extents on a drive	pvdisplay
logical volume group	multiple physical volume groups, one or more disks	lvdisplay

The following table provides an overview of some of the commands used in LVM and the functions they service.

LVM Commands

Command	LVM Function
vgcreate	Create a Volume Group. (Create a subset of the overall LVM)
pvcreate	Create a Physical Volume, assign to Volume Group. (Specify a disk for inclusion in the overall LVM)
vgextend	Add a new physical disk to a volume group.
lvcreate	Create a Logical Volume. (Storage area for related files that is part of a Volume Group. A Volume Group consists of many logical volumes)
lvextend	Increase the size of a Logical Volume.
lvreduce	Decrease the size of a Logical Volume.
lvremove	Removes a Logical Volume. (Frees the storage area set aside for a logical volume)
vgreduce	Reduce a Volume Group. (Reduces the number of disks in a Volume Group)
vgremove	Remove a Volume Group. (Removes the designation of a group of disks as a Volume Group)
vgdisplay	Volume Group Display. (Displays information about one or more Volume Groups)
pvdisplay	Physical Volume Display. (Displays information about one or more Volume Groups)
lvdisplay	Logical Volume Display. (Displays information about one or more Logical Volumes)

Create Logical Volumes

The following subsections describe the commands used for creating logical volumes.

Partition Types

Before using a hard disk as a physical volume, decide if the physical volume will use the entire disk (/dev/sdc) or a disk partition (/dev/sdc1).

To create the physical volume using a partition, set the partition type to 0x8e (Linux LVM) using fdisk or some other similar program.

Each logical volume in this section will use the entire hard disk. This requires that no partition table exists on the disk. When using the whole disk, the partition table must be erased, which will effectively destroy all data on that disk. An existing partition table can be removed by zeroing the first sector on the disk using the dd command.

[root@testnode1 ~]# dd if=/dev/zero of=/dev/sdc bs=512 count=1 1+0 records in 1+0 records out 512 bytes (512 B) copied, 0.00379734 seconds, 135 kB/s [root@testnode1 ~]# dd if=/dev/zero of=/dev/sdd bs=512 count=1 1+0 records in 1+0 records out 512 bytes (512 B) copied, 0.00200099 seconds, 256 kB/s

Verify that the partition table has been removed from both hard disks.

[root@testnode1 ~]# fdisk -l | grep '^Disk \/dev\/sd[cd][^:]' Disk /dev/sdc doesn't contain a valid partition table Disk /dev/sdd doesn't contain a valid partition table Disk /dev/dm-0 doesn't contain a valid partition table Disk /dev/dm-1 doesn't contain a valid partition table

Initialize Physical Volumes

Use the pvcreate command to initialize a block device to be used as a physical volume. The following commands will initialize the whole disk for each hard disk.

[root@testnode1 ~]# pvcreate /dev/sdc Physical volume "/dev/sdc" successfully created [root@testnode1 ~]# pvcreate /dev/sdd Physical volume "/dev/sdd" successfully created

Use the lvmdiskscan command to scan for block devices and verify that the two hard disks can be used as physical volumes.

[root@testnode1 ~]# lvmdiskscan | grep '\/dev\/sd[cd]' /dev/sdc [ 36.00 GB] LVM physical volume /dev/sdd [ 36.00 GB] LVM physical volume

Create Volume Group

Use the vgcreate command to create a volume group from one or more physical volumes. The vgcreate command creates a new volume group by name (vg_oradata and vg_orafra for example) and adds at least one physical volume to it. Create a new volume group on each hard disk.

[root@testnode1 ~]# vgcreate vg_oradata /dev/sdc Volume group "vg_oradata" successfully created [root@testnode1 ~]# vgcreate vg_orafra /dev/sdd Volume group "vg_orafra" successfully created

This creates a volume group descriptor at the start of each disk. When using partitions, run the vgcreate command on the partition (for example, vgcreate vg_oradata /dev/sdc1) which will create a volume group descriptor at the start of the partition.

Create Logical Volumes

With the new volume groups in place, use the lvcreate command to create the appropriate logical volumes (lv_oradata and lv_orafra). Logical volumes can be created as linear volumes, striped volumes, and mirrored volumes. For the purpose of this example, create a single linear volume within each of the volume groups that uses all of the unallocated space within the volume group.

[root@testnode1 ~]# lvcreate -l 100%FREE -n lv_oradata vg_oradata Logical volume "lv_oradata" created [root@testnode1 ~]# lvcreate -l 100%FREE -n lv_orafra vg_orafra Logical volume "lv_orafra" created

Verify LVM Configuration

Use the vgscan and lvscan commands to verify the new volume groups and logical volumes respectively.

[root@testnode1 ~]# vgscan Reading all physical volumes. This may take a while... Found volume group "vg_orafra" using metadata type lvm2 Found volume group "vg_oradata" using metadata type lvm2 Found volume group "vg_orasoftware" using metadata type lvm2 Found volume group "VolGroup00" using metadata type lvm2 [root@testnode1 ~]# lvscan ACTIVE '/dev/vg_orafra/lv_orafra' [36.00 GB] inherit ACTIVE '/dev/vg_oradata/lv_oradata' [36.00 GB] inherit ACTIVE '/dev/vg_orasoftware/lv_orasoftware' [35.97 GB] inherit ACTIVE '/dev/VolGroup00/LogVol00' [30.97 GB] inherit ACTIVE '/dev/VolGroup00/LogVol01' [4.91 GB] inherit

LVM volume groups and underlying logical volumes are included in the device special file directory tree in the /dev directory with the following layout.

  /dev/vg/lv/

For example:

  /dev/vg_oradata/lv_oradata
  /dev/vg_orafra/lv_orafra

Delete Logical Volumes

Before describing how to drop physical and logical volumes, note that it is very common to only want to remove a logical volume from a volume group. For example, if users no longer need the logical volume lv_orafra. The logical volume can be removed and its physical extents placed back in the empty pool for the volume group. First, the file system will need to be unmounted (if it is mounted). Next, deactive it with lvchange and finally delete it with lvremove. Here is an example that removes the lv_orafra.

[root@testnode1 ~]# umount /u04/oracle/fast_recovery_area [root@testnode1 ~]# lvchange -a n /dev/vg_orafra/lv_orafra [root@testnode1 ~]# lvremove /dev/vg_orafra/lv_orafra lvremove -- do you really want to remove "/dev/vg_orafra/lv_orafra"? [y/n]: y lvremove -- doing automatic backup of volume group "vg_orafra" lvremove -- logical volume "/dev/vg_orafra/lv_orafra" successfully removed

Use the following to delete all physical and logical volumes created in this guide.

[root@testnode1 ~]# umount /u03/oracle/oradata [root@testnode1 ~]# lvchange -a n /dev/vg_oradata/lv_oradata [root@testnode1 ~]# lvremove /dev/vg_oradata/lv_oradata [root@testnode1 ~]# vgchange -a n /dev/vg_oradata [root@testnode1 ~]# vgchange -a n /dev/vg_orafra [root@testnode1 ~]# vgremove /dev/vg_oradata [root@testnode1 ~]# vgremove /dev/vg_orafra

About the Author

Jeffrey Hunter is an Oracle Certified Professional, Java Development Certified Professional, Author, and an Oracle ACE. Jeff currently works as a Senior Database Administrator for The DBA Zone, Inc. located in Pittsburgh, Pennsylvania. His work includes advanced performance tuning, Java and PL/SQL programming, developing high availability solutions, capacity planning, database security, and physical / logical database design in a UNIX, Linux, and Windows server environment. Jeff's other interests include mathematical encryption theory, programming language processors (compilers and interpreters) in Java and C, LDAP, writing web-based database administration tools, and of course Linux. He has been a Sr. Database Administrator and Software Engineer for over 17 years and maintains his own website site at: http://www.iDevelopment.info. Jeff graduated from Stanislaus State University in Turlock, California, with a Bachelor's degree in Computer Science.