RH134 - ch10s03

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Managing Logical Volumes

pvcreate, pvremove, and pvdisplay create, remove, and list physical volumes (PV).
vgcreate, vgremove, and vgdisplay create, remove, and list volume groups (VG).
lvcreate, lvremove, and lvdisplay create, remove, and list logical volumes (LV).
Adding logical volumes is done in the order PV, VG, and LV.
Removing logical volumes is done in the order LV, VG, and PV.

Objectives

After completing this section, students should be able to:

Implement LVM storage.
Display LVM component information.

Implementing LVM storage

LVM comes with a comprehensive set of command-line tools for implementing and managing LVM storage. These command-line tools can be used in scripts, making them suitable for automation.

Important

The following examples use device vda and its partitions to illustrate LVM commands. In practice, these examples would need to use the correct devices for the disk and disk partitions that are being used by the system.

Creating a logical volume

Creating a logical volume

There are five steps needed to create a usable logical volume:

Prepare the physical device.
Use fdisk, gdisk or parted to create a new partition for use with LVM. Always set the partition type to Linux LVM on LVM partitions; use 0x8e for MBR-style partitions. If necessary, use partprobe to register the new partition with the kernel.
Alternatively, use a whole disk, a RAID array, or a SAN disk.
A physical device only needs to be prepared if there are none prepared already and a new physical volume is required to create or extend a volume group.
```
[root@serverX ~]# fdisk /dev/vda
```
Use m for help, p to print the existing partition table, n to create a new partition, t to change the partition type, w to write the changes, and q to quit.
Create a physical volume.
pvcreate is used to label the partition (or other physical device) for use with LVM as a physical volume. A header to store LVM configuration data is written directly to the PV. A PV is divided into physical extents (PE) of a fixed size; for example, 4MiB blocks. Label multiple devices at the same time by using space-delimited device names as arguments to pvcreate.
```
[root@serverX ~]# pvcreate /dev/vda2 /dev/vdb1
```
This will label devices /dev/vda2 and /dev/vdb1 as PVs, ready for allocation into a volume group.
A PV only needs to be created if there are no PVs free to create or extend a VG.
Create a volume group.
vgcreate is used to create a pool of one or more physical volumes, called a volume group. The size of the VG is determined by the total number of physical extents in the pool. A VG is responsible for hosting one or more logical volumes by allocating free PEs to a LV; therefore, it must have sufficient free PEs available at the time the LV is created.
As arguments to vgcreate, define a VG name and list one or more PVs to allocate to the VG.
```
[root@serverX ~]# vgcreate vg-alpha /dev/vda2 /dev/vdb1
```
This will create a VG called vg-alpha that is the combined size, in PE units, of the two PVs /dev/vda2 and /dev/vdb1.
A VG only needs to be created when there is none in existence. Additional VGs may be created for administrative reasons to manage the use of PVs and LVs. Otherwise, existing VGs can be extended to accommodate new LVs when needed.
Create a logical volume.
lvcreate creates a new logical volume from the available physical extents in a volume group. Use these arguments to lvcreate as a minimum: use the -n option to set the LV name, the -L option to set the LV size in bytes, and identify the VG name that the LV is to be created in.
```
[root@serverX ~]# lvcreate -n hercules -L 2G vg-alpha
```
This will create a LV called hercules, 2GiB in size, in the VG vg-alpha. There must be sufficient free physical extents to allocate 2GiB, and if necessary, it will be rounded to a factor of the PE unit size.
There are multiple ways to specify the size: -L expects sizes in bytes, or larger named values, such as mebibytes (binary megabytes, 1048576 bytes) and gibibytes (binary gigabytes). The -l option expects sizes measured as a number of physical extents.
Some examples:
- lvcreate -L 128M: Size the logical volume to exactly 128MiB.
- lvcreate -l 128 : Size the logical volume to exactly 128 extents in size. The total number of bytes depends on the size of the physical extent block on the underlying physical volume.
Important
Different tools will display the logical volume name using either the traditional name, /dev/vgname/lvname, or the kernel device mapper name, /dev/mapper/vgname-lvname.
Add the file system.
Use mkfs to create an xfs file system on the new logical volume. Alternatively, create a file system based on your preferred file system; for example, ext4.
```
[root@serverX ~]# mkfs -t xfs /dev/vg-alpha/hercules
```
To make the file system available across reboots:
- Use mkdir to create a mount point directory.
```
[root@serverX ~]# mkdir /mnt/hercules
```
- Add an entry to the /etc/fstab file:
```
/dev/vg-alpha/hercules  /mnt/hercules  xfs  defaults 1 2
```
- Run mount -a to mount all the file systems in /etc/fstab, including the entry just added.
```
[root@serverX ~]# mount -a
```

Removing a logical volume

Removing a logical volume

There are four steps needed to remove all logical volume components:

Prepare the file system.
Move all data that must be kept to another file system, then use umount to unmount the file system. Do not forget to remove any /etc/fstab entries associated with this file system.
```
[root@serverX ~]# umount /mnt/hercules
```
Warning
Removing a logical volume will destroy any data stored on the logical volume. Back up or move your data BEFORE you remove the logical volume.
Remove the logical volume.
lvremove is used to remove a logical volume that is no longer needed. Use the device name as the argument.
```
[root@serverX ~]# lvremove /dev/vg-alpha/hercules
```
The LV file system must be unmounted before running this command. It will ask for confirmation before removing the LV.
The LV's physical extents will be freed and made available for assignment to existing or new LVs in the volume group.
Remove the volume group.
vgremove is used to remove a volume group that is no longer needed. Use the VG name as the argument.
```
[root@serverX ~]# vgremove vg-alpha
```
The VG's physical volumes will be freed and made available for assignment to existing or new VGs on the system.
Remove the physical volumes.
pvremove is used to remove physical volumes that are no longer needed. Use a space-delimited list of PV devices to remove more than one at a time. The PV metadata is wiped from the partition (or disk). The partition is now free for reallocation or reformatting.
```
[root@serverX ~]# pvremove /dev/vda2 /dev/vdb1
```

Reviewing LVM status information

Physical volumes

Use pvdisplay to display information about physical volumes. If no arguments are specified with the command, it will list information about all PVs on the system. If the argument is a specific device name, then display information will be limited to that specific PV.

[root@serverX ~]# pvdisplay /dev/vda2
  --- Physical volume ---
  PV Name               /dev/vda2                        
  VG Name               vg-alpha                         
  PV Size               256.00 MiB / not usable 4.00 MiB 
  Allocatable           yes
  PE Size               4.00 MiB                         
  Total PE              63
  Free PE               26                               
  Allocated PE          37
  PV UUID               JWzDpn-LG3e-n2oi-9Etd-VT2H-PMem-1ZXwP1

	`PV Name` maps to the device name.
	`VG Name` shows the volume group where the PV is allocated.
	`PV Size` shows the physical size of the PV, including any unusable space.
	`PE Size` is the physical extent size, which is the smallest size a logical volume can be allocated. It is also the multiplying factor when calculating the size of any value reported in PE units, such as Free PE; for example: 26 PEs x 4MiB (the PE Size) gives 104MiB of free space. A logical volume size will be rounded to a factor of PE units. LVM sets the PE size automatically, although it is possible to specify it.
	`Free PE` shows how many PE units are available for allocation to new logical volumes.

Volume groups

Use vgdisplay to display information about volume groups. If no argument is specified for the command, then it will display information about all VGs. Using the VG name as an argument will limit the display information to that specific VG.

[root@serverX ~]# vgdisplay vg-alpha
  --- Volume group --- 
  VG Name               vg-alpha        
  System ID             
  Format                lvm2
  Metadata Areas        3  
  Metadata Sequence No  4    
  VG Access             read/write 
  VG Status             resizable   
  MAX LV                0            
  Cur LV                1
  Open LV               1
  Max PV                0
  Cur PV                3
  Act PV                3
  VG Size               1012.00 MiB       
  PE Size               4.00 MiB
  Total PE              253              
  Alloc PE / Size       175 / 700.00 MiB
  Free  PE / Size       78 / 312.00 MiB  
  VG UUID               3snNw3-CF71-CcYG-Llk1-p6EY-rHEv-xfUSez

	`VG Name` is the name of this volume group.
	`VG Size` is the total size of the storage pool available for logical volume allocation.
	`Total PE` is the total size expressed in PE units.
	`Free PE / Size` shows how much space is free in the VG for allocating to new LVs or to extend existing LVs.

Logical volumes

Use lvdisplay to display information about logical volumes. Again, no argument with the command will display information about all LVs, and using the LV device name as an argument will display information about that specific device.

[root@serverX ~]# lvdisplay /dev/vg-alpha/hercules
  --- Logical volume ---
  LV Path                /dev/vg-alpha/hercules 
  LV Name                hercules
  VG Name                vg-alpha               
  LV UUID                5IyRea-W8Zw-xLHk-3h2a-IuVN-YaeZ-i3IRrN
  LV Write Access        read/write
  LV Creation host, time server1.example.com 2014-02-19 00:26:48 -0500
  LV Status              available
  # open                 1
  LV Size                700 MiB                
  Current LE             175                    
  Segments               3
  Allocation             inherit
  Read ahead sectors     auto
  - current set to       8192
  Block device           252:0

	`LV Path` shows the device name of this logical volume. Some tools may report the device name as `/dev/mapper/vgname-lvname`; both represent the same LV.
	`VG Name` shows the volume group the LV is allocated from.
	`LV Size` shows the total size of the LV. Use file system tools to check free space and used space for storage of data.
	`Current LE` shows the number of logical extents used by this LV. A LE usually maps to a physical extent in the VG, and therefore the physical volume.

References

lvm(8), pvcreate(8), vgcreate(8), lvcreate(8), pvremove(8), vgremove(8), lvremove(8), pvdisplay(8), vgdisplay(8), lvdisplay(8), fdisk(8), gdisk(8), parted(8), partprobe(8), and mkfs(8) man pages

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Revision: rh134-7-63a207e

Red Hat System Administration II

Managing Logical Volumes

Objectives

Implementing LVM storage

Important

Important

Warning

Reviewing LVM status information

References