Mirrored Logical Volumes; Snapshot Volumes - Red Hat ENTERPRISE LINUX 5.1 - LVM ADMINISTRATION Manual

Chapter 2. LVM Components

2.3.3. Mirrored Logical Volumes

A mirror maintains identical copies of data on different devices. When data is written to one device, it
is written to a second device as well, mirroring the data. This provides protection for device failures.
When one leg of a mirror fails, the logical volume becomes a linear volume and can still be accessed.
LVM supports mirrored volumes. When you create a mirrored logical volume, LVM ensures that data
written to an underlying physical volume is mirrored onto a separate physical volume. With LVM, you
can create mirrored logical volumes with multiple mirrors.
An LVM mirror divides the device being copied into regions that are typically 512KB in size. LVM
maintains a small log which it uses to keep track of which regions are in sync with the mirror or
mirrors. This log can be kept on disk, which will keep it persistent across reboots, or it can be
maintained in memory.
Figure 2.6, "Mirrored Logical Volume"
configuration, the log is maintained on disk.
Figure 2.6. Mirrored Logical Volume
Note
Mirrored logical volumes are not currently supported in a cluster.
For information on creating and modifying mirrors, see

2.3.4. Snapshot Volumes

The LVM snapshot feature provides the ability to create virtual images of a device at a particular
instant without causing a service interruption. When a change is made to the original device (the
origin) after a snapshot is taken, the snapshot feature makes a copy of the changed data area as it
was prior to the change so that it can reconstruct the state of the device.
10
shows a mirrored logical volume with one mirror. In this
Section 4.4.1.3, "Creating Mirrored Volumes".