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Wednesday, November 4, 2015

Explain about the dmsetup Command in Linux?

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.

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:

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.

Possible device states are SUSPENDED, ACTIVE, and READ-ONLY. The dmsetup suspend command sets a device state to SUSPENDED.
When a device is suspended, all I/O operations to that device stop. The dmsetup resume command restores a device state to ACTIVE.

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 --readahead option of the dmsetup command.

Tables present:
Possible states for this category are LIVE and INACTIVE. An INACTIVE state indicates that a table has been loaded which will be swapped in when a dmsetup resume command restores a device state to ACTIVE, at which point the table's state becomes LIVE. For information, see the dmsetup man page.

Open count:
    The open reference count indicates how many times the device is opened. A mount command opens a device.

Event number:
The current number of events received. Issuing a dmsetup wait n command 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 frag ments 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 of the device.

The following example shows partial output for the dmsetup info command.

[root@testserver ~]# dmsetup info
Name:                       VolGroup00-LogVol01
State:                         ACTIVE
Read Ahead:             256
Tables present:          LIVE
Open count:              2
Event number:          0
Major, minor:            252, 1
Number of targets:    1
UUID: LVM-ZfQCQ1suTcykV9TwvNACpBXcEMNuWlnELoJOLg5TDC5ityl5a6qLJ5fujuoRRzWb

Name:                     VolGroup00-LogVol00
State:                      ACTIVE
Read Ahead:          256
Tables present:       LIVE
Open count:           1
Event number:       0
Major, minor:          252, 0
Number of targets: 1

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