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GMULTIPATH(8)		  BSD System Manager's Manual		 GMULTIPATH(8)

     gmultipath	-- disk	multipath control utility

     gmultipath	create [-ARv] name prov	...
     gmultipath	label [-ARv] name prov ...
     gmultipath	configure [-APRv] name
     gmultipath	add [-v] name prov
     gmultipath	remove [-v] name prov
     gmultipath	fail [-v] name prov
     gmultipath	restore	[-v] name prov
     gmultipath	rotate [-v] name
     gmultipath	prefer [-v] name prov
     gmultipath	getactive [-v] name
     gmultipath	destroy	[-v] name
     gmultipath	stop [-v] name
     gmultipath	clear [-v] prov	...
     gmultipath	list
     gmultipath	status
     gmultipath	load
     gmultipath	unload

     The gmultipath utility is used for	device multipath configuration.

     The multipath device can be configured using two different	methods:
     "manual" or "automatic".  When using the "manual" method, no metadata are
     stored on the devices, so the multipath device has	to be configured by
     hand every	time it	is needed.  Additional device paths also won't be de-
     tected automatically.  The	"automatic" method uses	on-disk	metadata to
     detect device and all it's	paths.	Metadata use the last sector of	the
     underlying	disk device and	include	device name and	UUID.  The UUID	guar-
     antees uniqueness in a shared storage environment but is in general too
     cumbersome	to use.	 The name is what is exported via the device inter-

     The first argument	to gmultipath indicates	an action to be	performed:

     create   Create multipath device with "manual" method without writing any
	      on-disk metadata.	 It is up to administrator, how	to properly
	      identify device paths.  Kernel will only check that all given
	      providers	have same media	and sector sizes.

	      -A option	enables	Active/Active mode, -R option enables Ac-
	      tive/Read	mode, otherwise	Active/Passive mode is used by de-

     label    Create multipath device with "automatic" method.	Label the
	      first given provider with	on-disk	metadata using the specified
	      name.  The rest of given providers will be retasted to detect
	      these metadata.  It reliably protects against specifying unre-
	      lated providers.	Providers with no matching metadata detected
	      will not be added	to the device.

	      -A option	enables	Active/Active mode, -R option enables Ac-
	      tive/Read	mode, otherwise	Active/Passive mode is used by de-

	      Configure	the given multipath device.

	      -A option	enables	Active/Active mode, -P option enables Ac-
	      tive/Passive mode, -R option enables Active/Read mode.

     add      Add the given provider as	a path to the given multipath device.
	      Should normally be used only for devices created with "manual"
	      method, unless you know what you are doing (you are sure that it
	      is another device	path, but tasting its metadata in regular
	      "automatic" way is not possible).

     remove   Remove the given provider	as a path from the given multipath de-
	      vice.  If	the last path removed, the multipath device will be

     fail     Mark specified provider as a path	of the specified multipath de-
	      vice as failed.  If there	are other paths	present, new requests
	      will be forwarded	there.

     restore  Mark specified provider as a path	of the specified multipath de-
	      vice as operational, allowing it to handle requests.

     rotate   Change the active	provider/path to the next available provider
	      in Active/Passive	mode.

     prefer   Change the active	provider/path to the specified provider	in Ac-
	      tive/Passive mode.

	      Get the currently	active provider(s)/path(s).

     destroy  Destroy the given	multipath device clearing metadata.

     stop     Stop the given multipath device without clearing metadata.

     clear    Clear metadata on	the given provider.

     list     See geom(8).

     status   See geom(8).

     load     See geom(8).

     unload   See geom(8).

     The following sysctl(8) variable can be used to control the behavior of
     the MULTIPATH GEOM	class.

     kern.geom.multipath.debug:	0
	     Debug level of the	MULTIPATH GEOM class.  This can	be set to 0
	     (default) or 1 to disable or enable various forms of chattiness.

     kern.geom.multipath.exclusive: 1
	     Open underlying providers exclusively, preventing individual
	     paths access.

     Exit status is 0 on success, and 1	if the command fails.

     This is a multiple	path architecture with no device knowledge or presump-
     tions other than size matching built in.  Therefore the user must exer-
     cise some care in selecting providers that	do indeed represent multiple
     paths to the same underlying disk device.	The reason for this is that
     there are several criteria	across multiple	underlying transport types
     that can indicate identity, but in	all respects such identity can rarely
     be	considered definitive.

     For example, if you use the World Word Port Name of a Fibre Channel disk
     object you	might believe that two disks that have the same	WWPN on	dif-
     ferent paths (or even disjoint fabrics) might be considered the same
     disk.  Nearly always this would be	a safe assumption, until you realize
     that a WWPN, like an Ethernet MAC address,	is a soft programmable entity,
     and that a	misconfigured Director Class switch could lead you to believe
     incorrectly that you have found multiple paths to the same	device.	 This
     is	an extreme and theoretical case, but it	is possible enough to indicate
     that the policy for deciding which	of multiple pathnames refer to the
     same device should	be left	to the system operator who will	use tools and
     knowledge of their	own storage subsystem to make the correct configura-
     tion selection.

     There are Active/Passive, Active/Read and Active/Active operation modes
     supported.	 In Active/Passive mode	only one path has I/O moving on	it at
     any point in time.	 This I/O continues until an I/O is returned with a
     generic I/O error or a "Nonexistent Device" error.	 When this occurs,
     that path is marked FAIL, the next	path in	a list is selected as active
     and the failed I/O	reissued.  In Active/Active mode all paths not marked
     FAIL may handle I/O same time.  Requests are distributed between paths to
     equalize load.  For capable devices it allows to utilize bandwidth	of all
     paths.  In	Active/Read mode all paths not marked FAIL may handle reads
     same time,	but unlike Active/Active only one path handles write requests
     at	any point in time.  It allows to closer	follow original	write request
     order if above layer needs	it for data consistency	(not waiting for req-
     uisite write completion before sending dependent write).

     When new devices are added	to the system the MULTIPATH GEOM class is
     given an opportunity to taste these new devices.  If a new	device has a
     MULTIPATH on-disk metadata	label, the device is used to either create a
     new MULTIPATH GEOM, or been added the list	of paths for an	existing

     It	is this	mechanism that works reasonably	with isp(4) and	mpt(4) based
     Fibre Channel disk	devices.  For these devices, when a device disappears
     (due e.g.,	to a cable pull	or power failure to a switch), the device is
     proactively marked	as gone	and I/O	to it failed.  This causes the
     MULTIPATH failure event just described.

     When Fibre	Channel	events inform either isp(4) or mpt(4) host bus
     adapters that new devices may have	arrived	(e.g., the arrival of an RSCN
     event from	the Fabric Domain Controller), they can	cause a	rescan to oc-
     cur and cause the attachment and configuration of any (now) new devices
     to	occur, causing the taste event described above.

     This means	that this multipath architecture is not	a one-shot path
     failover, but can be considered to	be steady state	as long	as failed
     paths are repaired	(automatically or otherwise).

     Automatic rescanning is not a requirement.	 Nor is	Fibre Channel.	The
     same failover mechanisms work equally well	for traditional	"Parallel"
     SCSI but may require manual intervention with camcontrol(8) to cause the
     reattachment of repaired device links.

     The following example shows how to	use camcontrol(8) to find possible
     multiple path devices and to create a MULTIPATH GEOM class	for them.

	   mysys# camcontrol devlist
	   <ECNCTX @WESTVILLE >	  at scbus0 target 0 lun 0 (da0,pass0)
	   <ECNCTX @WESTVILLE >	  at scbus0 target 0 lun 1 (da1,pass1)
	   <ECNCTX @WESTVILLE >	  at scbus1 target 0 lun 0 (da2,pass2)
	   <ECNCTX @WESTVILLE >	  at scbus1 target 0 lun 1 (da3,pass3)
	   mysys# camcontrol inquiry da0 -S
	   mysys# camcontrol inquiry da2 -S

     Now that you have used the	Serial Number to compare two disk paths	it is
     not entirely unreasonable to conclude that	these are multiple paths to
     the same device.  However,	only the user who is familiar with their stor-
     age is qualified to make this judgement.

     You can then use the gmultipath command to	label and create a MULTIPATH
     GEOM provider named FRED.

	   gmultipath label -v FRED /dev/da0 /dev/da2
	   disklabel -Brw /dev/multipath/FRED auto
	   newfs /dev/multipath/FREDa
	   mount /dev/multipath/FREDa /mnt....

     The resultant console output looks	something like:

	   GEOM_MULTIPATH: da0 added to	FRED
	   GEOM_MULTIPATH: da0 is now active path in FRED
	   GEOM_MULTIPATH: da2 added to	FRED

     geom(4), isp(4), mpt(4), loader.conf(5), camcontrol(8), geom(8),
     mount(8), newfs(8), sysctl(8)

     Matthew Jacob <>	Alexander Motin	<>

BSD				April 18, 2012				   BSD


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