4 LVM Configuration

LVM enables flexible distribution of hard disk space over several file systems. It was developed because the need to change the segmentation of hard disk space might arise only after the initial partitioning has already been done during installation. Because it is difficult to modify partitions on a running system, LVM provides a virtual pool (volume group or VG) of memory space from which logical volumes (LVs) can be created as needed. The operating system accesses these LVs instead of the physical partitions. Volume groups can span more than one disk, so that several disks or parts of them can constitute one single VG. In this way, LVM provides a kind of abstraction from the physical disk space that allows its segmentation to be changed in a much easier and safer way than through physical repartitioning.

Figure 4.1, “Physical Partitioning versus LVM” compares physical partitioning (left) with LVM segmentation (right). On the left side, one single disk has been divided into three physical partitions (PART), each with a mount point (MP) assigned so that the operating system can access them. On the right side, two disks have been divided into two and three physical partitions each. Two LVM volume groups (VG 1 and VG 2) have been defined. VG 1 contains two partitions from DISK 1 and one from DISK 2. VG 2 contains the remaining two partitions from DISK 2.

Figure 4.1: Physical Partitioning versus LVM #

In LVM, the physical disk partitions that are incorporated in a volume group are called physical volumes (PVs). Within the volume groups in Figure 4.1, “Physical Partitioning versus LVM”, four logical volumes (LV 1 through LV 4) have been defined, which can be used by the operating system via the associated mount points. The border between different logical volumes need not be aligned with any partition border. See the border between LV 1 and LV 2 in this example.

LVM features:

With these features, using LVM already makes sense for heavily used home PCs or small servers. If you have a growing data stock, as in the case of databases, music archives, or user directories, LVM is especially useful. It allows file systems that are larger than the physical hard disk. Another advantage of LVM is that up to 256 LVs can be added. However, keep in mind that working with LVM is different from working with conventional partitions.

Starting from kernel version 2.6, LVM version 2 is available, which is downward-compatible with the previous LVM and enables the continued management of old volume groups. When creating new volume groups, decide whether to use the new format or the downward-compatible version. LVM 2 does not require any kernel patches. It makes use of the device mapper integrated in kernel 2.6. This kernel only supports LVM version 2. Therefore, when talking about LVM, this section always refers to LVM version 2.

You can manage new or existing LVM storage objects by using the YaST Partitioner. Instructions and further information about configuring LVM is available in the official LVM HOWTO (http://tldp.org/HOWTO/LVM-HOWTO/).

Important

If you add multipath support after you have configured LVM, you must modify the /etc/lvm/lvm.conf file to scan only the multipath device names in the /dev/disk/by-id directory as described in Section 7.2.4, “Using LVM2 on Multipath Devices”, then reboot the server.

For each disk, partition the free space that you want to use for LVM as 0x8E Linux LVM. You can create one or multiple LVM partitions on a single device. It is not necessary for all of the partitions on a device to be LVM partitions.

You can use the Volume Group function to group one or more LVM partitions into a logical pool of space called a volume group, then carve out one or more logical volumes from the space in the volume group.

In the YaST Partitioner, only the free space on the disk is made available to you as you are creating LVM partitions. If you want to use the entire disk for a single LVM partition and other partitions already exists on the disk, you must first remove all of the existing partitions to free the space before you can use that space in an LVM partition.

Warning

Deleting a partition destroys all of the data in the partition.

An LVM volume group organizes the Linux LVM partitions into a logical pool of space. You can carve out logical volumes from the available space in the group. The Linux LVM partitions in a group can be on the same or different disks. You can add LVM partitions from the same or different disks to expand the size of the group. Assign all partitions reserved for LVM to a volume group. Otherwise, the space on the partition remains unused.

When the Linux LVM partitions are assigned to a volume group, the partitions are then referred to as a physical volumes.

Figure 4.2: Physical Volumes in the Volume Group Named Home #

To add more physical volumes to an existing volume group:

After a volume group has been filled with physical volumes, use the Logical Volumes dialog box (see Figure 4.3, “Logical Volume Management”) to define and manage the logical volumes that the operating system should use. This dialog lists all of the logical volumes in that volume group. You can use Add, Edit, and Remove options to manage the logical volumes. Assign at least one logical volume to each volume group. You can create new logical volumes as needed until all free space in the volume group has been exhausted.

Beginning in SLES 11 SP3, an LVM logical volume can be thinly provisioned. Thin provisioning allows you to create logical volumes with sizes that overbook the available free space. You create a thin pool that contains unused space reserved for use with an arbitrary number of thin volumes. A thin volume is created as a sparse volume and space is allocated from a thin pool as needed. The thin pool can be expanded dynamically when needed for cost-effective allocation of storage space.

Important

To use thinly provisioned volumes in a cluster, the thin pool and the thin volumes that use it must be managed in a single cluster resource. This allows the thin volumes and thin pool to always be mounted exclusively on the same node.

Figure 4.3: Logical Volume Management #

It is possible to distribute the data stream in the logical volume among several physical volumes (striping). If these physical volumes reside on different hard disks, this generally results in a better reading and writing performance (like RAID 0). However, a striping LV with n stripes can only be created correctly if the hard disk space required by the LV can be distributed evenly to n physical volumes. For example, if only two physical volumes are available, a logical volume with three stripes is impossible.

Activation behavior for non-root LVM volume groups is controlled by parameter settings in the /etc/sysconfig/lvm file.

By default, non-root LVM volume groups are automatically activated on system restart by /etc/rc.d/boot.lvm, according to the setting for the LVM_VGS_ACTIVATED_ON_BOOT parameter in the /etc/sysconfig/lvm file. This parameter allows you to activate all volume groups on system restart, or to activate only specified non-root LVM volume groups.

To activate all non-root LVM volume groups on system restart, ensure that the value for the LVM_VGS_ACTIVATED_ON_BOOT parameter in the /etc/sysconfig/lvm file is empty (""). This is the default setting. For almost all standard LVM installations, it can safely stay empty.

LVM_VGS_ACTIVATED_ON_BOOT="" 

To activate only a specified non-root LVM volume group on system restart, specify the volume group name as the value for the LVM_VGS_ACTIVATED_ON_BOOT parameter:

LVM_VGS_ACTIVATED_ON_BOOT="vg1" 

By default, newly discovered LVM volume groups are not automatically activated. The LVM_ACTIVATED_ON_DISCOVERED parameter is disabled in the /etc/sysconfig/lvm file:

LVM_ACTIVATED_ON_DISCOVERED="disable"

You can enable the LVM_ACTIVATED_ON_DISCOVERED parameter to allow newly discovered LVM volume groups to be activated via udev rules:

LVM_ACTIVATED_ON_DISCOVERED="enable"

A tag is an unordered keyword or term assigned to the metadata of a storage object. Tagging allows you to classify collections of LVM storage objects in ways that you find useful by attaching an unordered list of tags to their metadata.

tags {
   # Enable hostname tags
   hosttags = 1
}

tags {

   tag1 { }
      # Tag does not require a match to be set.

   tag2 {
      # If no exact match, tag is not set.
      host_list = [ "hostname1", "hostname2" ]
   }
}

  activation {
      volume_list = [ "vg1/lvol0", "@database" ]
  }

You can add and remove Linux LVM partitions from a volume group to expand or reduce its size.

Warning

Removing a partition can result in data loss if the partition is in use by a logical volume.

The lvresize, lvextend, and lvreduce commands are used to resize logical volumes. See the man pages for each of these commands for syntax and options information.

You can also increase the size of a logical volume by using the YaST Partitioner. YaST uses parted(8) to grow the partition.

To extend an LV there must be enough unallocated space available on the VG.

LVs can be extended or shrunk while they are being used, but this may not be true for a file system on them. Extending or shrinking the LV does not automatically modify the size of file systems in the volume. You must use a different command to grow the file system afterwards. For information about resizing file systems, see Chapter 5, Resizing File Systems.

Ensure that you use the right sequence:

To extend the size of a logical volume:

For example, to extend an LV with a (mounted and active) ReiserFS on it by 10GB:

lvextend −L +10G /dev/vgname/lvname
resize_reiserfs −s +10GB −f /dev/vg−name/lv−name

For example, to shrink an LV with a ReiserFS on it by 5GB:

umount /mountpoint−of−LV
resize_reiserfs −s −5GB /dev/vgname/lvname
lvreduce /dev/vgname/lvname
mount /dev/vgname/lvname /mountpoint−of−LV

For information about using LVM commands, see the man pages for the commands described in Table 4.1, “LVM Commands”. Perform the commands as the root user.

Table 4.1: LVM Commands #

Command	Description
pvcreate <device>	Initializes a device (such as /dev/sdb) for use by LVLM as a physical volume.
pvdisplay <device>	Displays information about the LVM physical volume, such as whether it is currently being used in a logical volume.
vgcreate -c y <vg_name> <dev1> [dev2...]	Creates a clustered volume group with one or more specified devices.
vgchange -a [ey | n] <vg_name>	Activates (-a ey) or deactivates (-a n) a volume group and its logical volumes for input/output. Important Ensure that you use the ey option to exclusively activate a volume group on a cluster node. This option is used by default in the load script.
vgremove <vg_name>	Removes a volume group. Before using this command, remove the logical volumes, then deactivate the volume group.
vgdisplay <vg_name>	Displays information about a specified volume group. To find the total physical extent of a volume group, enter vgdisplay vg_name | grep "Total PE"
lvcreate -L size -n <lv_name> <vg_name>	Creates a logical volume of the specified size.
lvcreate -L <size> <-T|--thinpool> <vg_name/thin_pool_name> lvcreate -virtualsize <size> <-T|--thin> <vg_name/thin_pool_name> -n <thin_lv_name> lvcreate -L <size> -T <vg_name/thin_pool_name> -virtualsize <size> -n <thin_lv_name>	Creates a thin logical volume or a thin pool of the specified size. Creates thin pool or thin logical volume or both. Specifying the optional argument --size will cause the creation of the thin pool logical volume. Specifying the optional argument --virtualsize will cause the creation of the thin logical volume from given thin pool volume. Specifying both arguments will cause the creation of both thin pool and thin volume using this pool.
lvcreate -s [-L size] -n <snap_volume> <source_volume_path>	Creates a snapshot volume for the specified logical volume. If the size option (-L, --size) is not included, the snapshot is created as a thin snapshot.
lvremove</dev/vg_name/lv_name>	Removes a logical volume, such as /dev/vg_name/lv_name. Before using this command, close the logical volume by dismounting it with the umount command.
lvremove snap_volume_path	Removes a snapshot volume.
lvconvert --merge [-b] [-i <seconds>] [<snap_volume_path>[...<snapN>]|@<volume_tag>]	Reverts (rolls back or merges) the snapshot data into the original volume.
vgextend <vg_name><device>	Adds a specified physical volume to an existing volume group
vgreduce <vg_name> <device>	Removes a specified physical volume from an existing volume group. Important Ensure that the physical volume is not currently being used by a logical volume. If it is, you must move the data to another physical volume by using the pvmove command.
lvextend -L size</dev/vg_name/lv_name>	Extends the size of a specified logical volume. Afterwards, you must also expand the file system to take advantage of the newly available space.
lvreduce -L size </dev/vg_name/lv_name>	Reduces the size of a specified logical volume. Important Ensure that you reduce the size of the file system first before shrinking the volume, otherwise you risk losing data.
lvrename </dev/vg_name/old_lv_name> </dev/vg_name/new_lv_name>	Renames an existing LVM logical volume in a volume group from the old volume name to the new volume name. It does not change the volume group name.