4.4.1. Creating Logical Volumes

4.4.1. Creating Logical Volumes

To create a logical volume, use the lvcreate command. You can create linear volumes, striped volumes, and mirrored volumes, as described in the following subsections.

If you do not specify a name for the logical volume, the default name lvol# is used where # is the internal number of the logical volume.

The following sections provide examples of logical volume creation for the three types of logical volumes you can create with LVM.

4.4.1.1. Creating Linear Volumes

When you create a logical volume, the logical volume is carved from a volume group using the free extents on the physical volumes that make up the volume group. Normally logical volumes use up any space available on the underlying physical volumes on a next-free basis. Modifying the logical volume frees and reallocates space in the physical volumes.

The following command creates a logical volume 10 gigabytes in size in the volume group vg1.

lvcreate -L 10G vg1

The following command creates a 1500 megabyte linear logical volume named testlv in the volume group testvg, creating the block device /dev/testvg/testlv.

lvcreate -L1500 -ntestlv testvg

The following command creates a 50 gigabyte logical volume named gfslv from the free extents in volume group vg0.

lvcreate -L 50G -n gfslv vg0

You can use the -l argument of the lvcreate command to specify the size of the logical volume in extents. You can also use this argument to specify the percentage of the volume group to use for the logical volume. The following command creates a logical volume called mylv that uses 60% of the total space in volume group testvol.

lvcreate -l 60%VG -n mylv testvg

You can also use the -l argument of the lvcreate command to specify the percentage of the remaining free space in a volume group as the size of the logical volume. The following command creates a logical volume called yourlv that uses all of the unallocated space in the volume group testvol.

lvcreate -l 100%FREE -n yourlv testvg

You can use -l argument of the lvcreate command to create a logical volume that uses the entire volume group. Another way to create a logical volume that uses the entire volume group is to use the vgdisplay command to find the "Total PE" size and to use those results as input to the the lvcreate command.

The following commands create a logical volume called mylv that fills the volume group named testvg.

# vgdisplay testvg | grep "Total PE"
Total PE              10230
# lvcreate -l 10230 testvg -n mylv

The underlying physical volumes used to create a logical volume can be important if the physical volume needs to be removed, so you may need to consider this possibility when you create the logical volume. For information on removing a physical volume from a volume group, see Section 4.3.5, “Removing Physical Volumes from a Volume Group”.

To create a logical volume to be allocated from a specific physical volume in the volume group, specify the physical volume or volumes at the end at the lvcreate command line. The following command creates a logical volume named testlv in volume group testvg allocated from the physical volume /dev/sdg1,

lvcreate -L 1500 -ntestlv testvg /dev/sdg1

You can specify which extents of a physical volume are to be used for a logical volume. The following example creates a linear logical volume out of extents 0 through 25 of physical volume /dev/sda1 and extents 50 through 125 of physical volume /dev/sdb1 in volume group testvg.

lvcreate -l 100 -n testlv testvg /dev/sda1:0-25 /dev/sdb1:50-125

The following example creates a linear logical volume out of extents 0 through 25 of physical volume /dev/sda1 and then continues laying out the logical volume at extent 100.

lvcreate -l 100 -n testlv testvg /dev/sda1:0-25:100-

The default policy for how the extents of a logical volume are allocated is inherit, which applies the same policy as for the volume group. These policies can be changed using the lvchange command. For information on allocation policies, see Section 4.3.1, “Creating Volume Groups”.

4.4.1.2. Creating Striped Volumes

For large sequential reads and writes, creating a striped logical volume can improve the efficiency of the data I/O. For general information about striped volumes, see Section 2.3.2, “Striped Logical Volumes”.

When you create a striped logical volume, you specify the number of stripes with the -i argument of the lvcreate command. This determines over how many physical volumes the logical volume will be striped. The number of stripes cannot be greater than the number of physical volumes in the volume group (unless the --alloc anywhere argument is used).

The stripe size should be tuned to a power of 2 between 4kB and 512kB, and matched to the application's I/O that is using the striped volume. The -I argument of the lvcreate command specifies the stripe size in kilobytes.

If the underlying physical devices that make up a striped logical volume are different sizes, the maximum size of the striped volume is determined by the smallest underlying device. For example, in a two-legged stripe, the maximum size is twice the size of the smaller device. In a three-legged stripe, the maximum size is three times the size of the smallest device.

The following command creates a striped logical volume across 2 physical volumes with a stride of 64kB. The logical volume is 50 gigabytes in size, is named gfslv, and is carved out of volume group vg0.

lvcreate -L 50G -i2 -I64 -n gfslv vg0

As with linear volumes, you can specify the extents of the physical volume that you are using for the stripe. The following command creates a striped volume 100 extents in size that stripes across two physical volumes, is named stripelv and is in volume group testvg. The stripe will use sectors 0-50 of /dev/sda1 and sectors 50-100 of /dev/sdb1.

# lvcreate -l 100 -i2 -nstripelv testvg /dev/sda1:0-50 /dev/sdb1:50-100
  Using default stripesize 64.00 KB
  Logical volume "stripelv" created

4.4.1.3. Creating Mirrored Volumes

When you create a mirrored volume, you specify the number of copies of the data to make with the -m argument of the lvcreate command. Specifying -m1 creates one mirror, which yields two copies of the file system: a linear logical volume plus one copy. Similarly, specifying -m2 creates two mirrors, yielding three copies of the file system.

The following command creates a mirrored logical volume with a single mirror. The volume is 50 gigabytes in size, is named mirrorlv, and is carved out of volume group vg0:

lvcreate -L 50G -m1 -n gfslv vg0

An LVM mirror divides the device being copied into regions that, by default, are 512KB in size. You can use the -R argument to specify the region size in MB. LVM maintains a small log which it uses to keep track of which regions are in sync with the mirror or mirrors. By default, this log is kept on disk, which keeps it persistent across reboots. You can specify instead that this log be kept in memory with the --corelog argument; this eliminates the need for an extra log device, but it requires that the entire mirror be resynchronized at every reboot.

The following command creates a mirrored logical volume from the volume group bigvg. The logical is named ondiskmirvol and has a single mirror. The volume is 12MB in size and keeps the mirror log in memory.

# lvcreate -L 12MB -m1 --corelog -n ondiskmirvol bigvg
  Logical volume "ondiskmirvol" created

When a mirror is created, the mirror regions are synchronized. For large mirror components, the sync process may take a long time. When you are creating a new mirror that does not need to be revived, you can specify the nosync argument to indicate that an initial synchronization from the first device is not required.

You can specify which devices to use for the mirror logs and log, and which extents of the devices to use. To force the log onto a particular disk, specify exactly one extent on the disk on which it will be placed. LVM does not necessary respect the order in which devices are listed in the command line. If any physical volumes are listed that is the only space on which allocation will take place. Any physical extents included in the list that are already allocated will get ignored.

The following command creates a mirrored logical volume with a single mirror. The volume is 500 megabytes in size, it is named mirrorlv, and it is carved out of volume group vg0. The first leg of the mirror is on device /dev/sda1, the second leg of the mirror is on device /dev/sdb1, and the mirror log is on /dev/sdc1.

lvcreate -L 500M -m1 -n mirrorlv vg0 /dev/sda1 /dev/sdb1 /dev/sdc1

The following command creates a mirrored logical volume with a single mirror. The volume is 500 megabytes in size, it is named mirrorlv, and it is carved out of volume group vg0. The first leg of the mirror is on extents 0 through 499 of device /dev/sda1, the second leg of the mirror is on extents 0 through 499 of device /dev/sdb1, and the mirror log starts on extent 0 of device /dev/sdc1. These are 1MB extents. If any of the specified extents have already been allocated, they will be ignored.

lvcreate -L 500M -m1 -n mirrorlv vg0 /dev/sda1:0-499 /dev/sdb1:0-499 /dev/sdc1:0

4.4.1.4. Changing Mirrored Volume Configuration

You can convert a logical volume from a mirrored volume to a linear volume or from a linear volume to a mirrored volume with the lvconvert command. You can also use this command to reconfigure other mirror parameters of an existing logical volume, such as corelog.

When you convert a logical volume to a mirrored volume, you are basically creating mirror legs for an existing volume. This means that your volume group must contain the devices and space for the mirror legs and for the mirror log.

If you lose a leg of a mirror, LVM converts the volume to a linear volume so that you still have access to the volume, without the mirror redundancy. After you replace the leg, you can use the lvconvert command to restore the mirror. This procedure is provided in Section 6.3, “Recovering from LVM Mirror Failure”.

The following command converts the linear logical volume vg00/lvol1 to a mirrored logical volume.

lvconvert -m1 vg00/lvol1

The following command converts the mirrored logical volume vg00/lvol1 to a linear logical volume, removing the mirror leg.

lvconvert -m0 vg00/lvol1

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