12-06-2009
When using SCSI devices, the path is clearly defined based on the SCSI ID of the controller, the port that the device is connected to, etc.
SATA/SAS drives do not have this luxury, as they are not defined on a chain. This causes some confusion, as when /dev/sda comes up next time, it may be /dev/sdc.
dev-mapper uses a different, non-volatile ID such as UUID or disk label (contained in metadata) to keep disk paths consistent, through its abstraction.
Basically, the short version is:
To make sure that your devices are named and in a consistent path, dev-mapper handles the devices so that your / is found properly each bootup and is not confused with /tmp or some other file system on another device.
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LEARN ABOUT CENTOS
dmevent_tool
DMEVENT_TOOL(8) DMEVENT_TOOL(8)
NAME
dmevent_tool - A utility used to load a DSO into dmeventd and (un)register devices with it for monitoring
SYNOPSIS
dmevent_tool -[Vhmru] {RAID device name} {DSO Name}
DESCRIPTION
dmevent_tool is a userspace utility used to register/unregister DSOs with the daemon dmeventd.
If used with the -m command line parameter users can display all of the actively dmeventd monitored devices.
OPTIONS
-V Show version of dmevent_tool
-{h/?} Show this help information
-m[r|u] List all currently active device mapper devices
and their current status with dmeventd
for registered (-r)/unregistered (-m) devices
Syntax: dmevent_tool -m[u|r]
-a[r|u] Same as -m, but for devices with UUID only!
Syntax: dmevent_tool -a[u|r]
-r Register a device with dmeventd
Syntax: dmevent_tool -r <device name> <path to DSO library>
Example: dmevent_tool -r isw_abcdeh_Volume0 libdmraid-events.so
-u Unregister a device with dmeventd
Syntax: dmevent_tool -u <device name> [<path to DSO library>]
Example: dmevent_tool -u isw_abcdefgh_Volume0
EXAMPLES
dmevent_tool -m is used to display all of the actively monitored devices
dmevent_tool -m
Device Name: isw_defeaigdde_Volume0_dmraid00
Registered DSO: libdmraid-events.so
UUID: isw_defeaigdde_Volume0_dmraid00
status: Active
major device #: 253
minor device #: 0
read only device: No
number of recorded kernel events: 0
SATA drives in this volume group: /dev/sdb /dev/sdc
dmevent_tool -r is used to register a volume group device with dmeventd
dmevent_tool -r isw_defeaigdde_Volume0_dmraid00 libdmraid-events.so
dmevent_tool -u is used to unregister a volume group from dmeventd
dmevent_tool -u isw_defeaigdde_Volume0_dmraid00
DIAGNOSTICS
dmevent_tool returns an exit code of 0 for success or 1 for error.
AUTHOR
Brian Wood <brian.j.wood@intel.com>
Brian Wood 1.0.0.rc3 DMEVENT_TOOL(8)