Using CLI Commands LVM
When sizes are required in a command line argument, units can always be specified explicitly. If you do not specify a unit, then a default is assumed, usually KB or MB. LVM CLI commands do not accept fractions.
When specifying units in a command line argument, LVM is case-insensitive; specifying M or m is equivalent, for example, and powers of 2 (multiples of 1024) are used. However, when specifying the –units argument in a command, lower-case indicates that units are in multiples of 1024 while upper-case indicates that units are in multiples of 1000.
All LVM commands accept a -v argument, which can be entered multiple times to increase the output verbosity. For example, the following examples shows the default output of the lvcreate command.
lvcreate -L 50MB new_vg
lvcreate command with the -v argument.
lvcreate -v -L 50MB new_vg
The LVM Configuration Files
LVM supports multiple configuration files. At system startup, the lvm.conf configuration file is loaded from the directory specified by the environment variable LVM_SYSTEM_DIR, which is set to /etc/lvm by default.
The lvm.conf file can specify additional configuration files to load. Settings in later files override settings from earlier ones. To display the settings in use after loading all the configuration files, execute the lvm dumpconfig command.
The following files are used for LVM configuration:
/etc/lvm/lvm.conf
Central configuration file read by the tools.
–
/etc/lvm/lvm_hosttag.conf
For each host tag, an extra configuration file is read if it exists: lvm_hosttag.conf.
LVM Architecture Overview
LVM2 is backwards compatible with LVM1, with the exception of snapshot and cluster support. You can convert a volume group from LVM1 format to LVM2 format with the vgconvert command. For information on converting LVM metadata format, see the vgconvert(8) man page.
The underlying physical storage unit of an LVM logical volume is a block device such as a partition or whole disk. This device is initialized as an LVM physical volume (PV).
To create an LVM logical volume, the physical volumes are combined into a volume group (VG). This creates a pool of disk space out of which LVM logical volumes (LVs) can be allocated. This process is analogous to the way in which disks are divided into partitions. A logical volume is used by file systems and applications (such as databases).
There are several general features of all LVM CLI commands.
When sizes are required in a command line argument, units can always be specified explicitly. If you do not specify a unit, then a default is assumed, usually KB or MB. LVM CLI commands do not accept fractions.
Where commands take volume group or logical volume names as arguments, the full path name is optional. A logical volume called lvol0 in a volume group called vg0 can be specified as vg0/lvol0. Where a list of volume groups is required but is left empty, a list of all volume groups will be substituted. Where a list of logical volumes is required but a volume group is given, a list of all the logical volumes in that volume group will be substituted. For example, the lvdisplay vg0 command will display all the logical volumes in volume group vg0.
All LVM commands accept a -v argument, which can be entered multiple times to increase the output verbosity. For example, the following examples shows the default output of the lvcreate command.
# lvcreate -L 50MB new_vg
Rounding up size to full physical extent 52.00 MB
Logical volume “lvol0″ created
The following command shows the output of the lvcreate command with the -v argument.
# lvcreate -v -L 50MB new_vg
Finding volume group “new_vg”
Rounding up size to full physical extent 52.00 MB
Archiving volume group “new_vg” metadata (seqno 4).
Creating logical volume lvol0
Creating volume group backup “/etc/lvm/backup/new_vg” (seqno 5).
Found volume group “new_vg”
Creating new_vg-lvol0
Loading new_vg-lvol0 table
Resuming new_vg-lvol0 (253:2)
Clearing start of logical volume “lvol0″
Creating volume group backup “/etc/lvm/backup/new_vg” (seqno 5).
Logical volume “lvol0″ created
All LVM objects are referenced internally by a UUID, which is assigned when you create the object.
More about LVM Components Physical Volumes
Physical Volumes
The underlying physical storage unit of an LVM logical volume is a block device such as a partition or whole disk. To use the device for an LVM logical volume the device must be initialized as a physical volume (PV).
Initializing a block device as a physical volume places a label near the start of the device.
By default, the LVM label is placed in the second 512-byte sector. You can overwrite this default by placing the label on any of the first 4 sectors. This allows LVM volumes to co-exist with other users of these sectors, if necessary.
An LVM label provides correct identification and device ordering for a physical device, since devices can come up in any order when the system is booted. An LVM label remains persistent across reboots and throughout a cluster.
The LVM label identifies the device as an LVM physical volume. It contains a random unique identifier (the UUID) for the physical volume. It also stores the size of the block device in bytes, and it records where the LVM metadata will be stored on the device.
The LVM metadata contains the configuration details of the LVM volume groups on your system. By default, an identical copy of the metadata is maintained in every metadata area in every physical volume within the volume group. LVM metadata is small and stored as ASCII.
