A.2. The dmsetup Command
The
dmsetup command is a command line wrapper for communication with the Device Mapper. For general system information about LVM devices, you may find the info, ls, status, and deps options of the dmsetup command to be useful, as described in the following subsections.
For information about additional options and capabilities of the
dmsetup command, see the dmsetup(8) man page.
A.2.1. The dmsetup info Command
The
dmsetup info device command provides summary information about Device Mapper devices. If you do not specify a device name, the output is information about all of the currently configured Device Mapper devices. If you specify a device, then this command yields information for that device only.
The
dmsetup info command provides information in the following categories:
Name- The name of the device. An LVM device is expressed as the volume group name and the logical volume name separated by a hyphen. A hyphen in the original name is translated to two hyphens. During standard LVM operations, you should not use the name of an LVM device in this format to specify an LVM device directly, but instead you should use the vg/lv alternative.
State- Possible device states are
SUSPENDED,ACTIVE, andREAD-ONLY. Thedmsetup suspendcommand sets a device state toSUSPENDED. When a device is suspended, all I/O operations to that device stop. Thedmsetup resumecommand restores a device state toACTIVE. Read Ahead- The number of data blocks that the system reads ahead for any open file on which read operations are ongoing. By default, the kernel chooses a suitable value automatically. You can change this value with the
--readaheadoption of thedmsetupcommand. Tables present- Possible states for this category are
LIVEandINACTIVE. AnINACTIVEstate indicates that a table has been loaded which will be swapped in when admsetup resumecommand restores a device state toACTIVE, at which point the table's state becomesLIVE. For information, see thedmsetupman page. Open count- The open reference count indicates how many times the device is opened. A
mountcommand opens a device. Event number- The current number of events received. Issuing a
dmsetup wait ncommand allows the user to wait for the n'th event, blocking the call until it is received. Major, minor- Major and minor device number
Number of targets- The number of fragments that make up a device. For example, a linear device spanning 3 disks would have 3 targets. A linear device composed of the beginning and end of a disk, but not the middle would have 2 targets.
UUID- UUID of the device.
The following example shows partial output for the
dmsetup info command.
# dmsetup info
Name: testgfsvg-testgfslv1
State: ACTIVE
Read Ahead: 256
Tables present: LIVE
Open count: 0
Event number: 0
Major, minor: 253, 2
Number of targets: 2
UUID: LVM-K528WUGQgPadNXYcFrrf9LnPlUMswgkCkpgPIgYzSvigM7SfeWCypddNSWtNzc2N
...
Name: VolGroup00-LogVol00
State: ACTIVE
Read Ahead: 256
Tables present: LIVE
Open count: 1
Event number: 0
Major, minor: 253, 0
Number of targets: 1
UUID: LVM-tOcS1kqFV9drb0X1Vr8sxeYP0tqcrpdegyqj5lZxe45JMGlmvtqLmbLpBcenh2L3
