What is kmpathd?

Why am I seeing “kmpathd” processes?

$
610 root 10130 0.0 0.0 0 0 ? S< 2013 0:00 [kmpathd/0]
611 root 10131 0.0 0.0 0 0 ? S< 2013 0:00 [kmpathd/1]
612 root 10132 0.0 0.0 0 0 ? S< 2013 0:00 [kmpathd/2]
.
.
.
671 root 10191 0.0 0.0 0 0 ? S< 2013 0:00 [kmpathd/61]
672 root 10192 0.0 0.0 0 0 ? S< 2013 0:00 [kmpathd/62]
673 root 10193 0.0 0.0 0 0 ? S< 2013 0:00 [kmpathd/63]

kmpathd and kmpath_handlerd are kernel workqueue processes defined in drivers/md/dm-mpath.c, as part of the “Device Mapper Multipath” feature of the kernel’s “md” (Multiple Devices) software RAID driver. The driver is a separate kernel module called dm-multipath, configured via /sbin/dmsetup and /sbin/multipath. When the module is loaded, you’ll see one ‘kmpathd’ kernel-process in “ps” for every CPU core.

Device Mapper Multipathing is not actually a Software RAID level- it combines multiple I/O paths over a SAN between server nodes and storage arrays into a single device. These I/O paths are physical SAN connections that can include separate cables, switches, and controllers. Multipathing aggregates the I/O paths, creating a new device that consists of the aggregated paths, and thus offers both redundancy and improved I/O performance.

View the available device-mapper driver targets:

$
# /sbin/dmsetup targets
multipath v1.0.6
raid45 v1.0.0
snapshot-origin v1.6.0
snapshot v1.6.0
zero v1.0.0
mirror v1.2.0
striped v1.1.0
linear v1.0.2
error v1.0.1
#

View current device-mapper mappings:
Here two physical disks (8:17, 8:2) are joined into a single linear block:

$
# /sbin/dmsetup table
VolGroup00-LogVol01: 0 323092480 linear 8:17 1221656960
VolGroup00-LogVol01: 323092480 209453056 linear 8:2 384
VolGroup00-LogVol00: 0 1221656576 linear 8:17 384
#

View mappings as a tree:

$
# /sbin/dmsetup ls --tree
VolGroup00-LogVol01 (253:1)
|- (8:2)
`- (8:17)
VolGroup00-LogVol00 (253:0)
`- (8:17)
#
$
# /sbin/dmsetup info
Name: VolGroup00-LogVol01
State: ACTIVE
Read Ahead: 256
Tables present: LIVE
Open count: 1
Event number: 0
Major, minor: 253, 1
Number of targets: 2
UUID: LVM-0iF8GRZyq4aRNBxTr8j5FOB4A887UpDx8GPGJ2DfjW6ziKZpZx5dV8XdO3FxKBpN
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-0iF8GRZyq4aRNBxTr8j5FOB4A887UpDx63J0ZhtZVVjwmFoXrkVf1omReP9ve8CT
#

See also

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