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

NAME
     gmultipath — disk multipath control utility

SYNOPSIS
     gmultipath label [-hv] name prov ...
     gmultipath clear [-v] prov ...
     gmultipath list
     gmultipath status
     gmultipath load
     gmultipath unload

DESCRIPTION
     The gmultipath utility is used for device multipath configuration.

     Only automatic configuration is supported at the present time via the
     label command.  This operation writes a label on the last sector of the
     underlying disk device with a contained name and UUID.  The UUID guaran‐
     tees uniqueness in a shared storage environment but is in general too
     cumbersome to use.	 The name is what is exported via the device inter‐
     face.

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

     label    Label the given underlying device with the specified name.  The
	      kernel module geom_multipath.ko will be loaded if it is not
	      loaded already.

     clear    Clear metadata on the given device.

     list     See geom(8).

     status   See geom(8).

     load     See geom(8).

     unload   See geom(8).

SYSCTL VARIABLES
     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.

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

MULTIPATH ARCHITECTURE
     This is an active/passive multiple path architecture with no device
     knowledge or presumptions other than size matching built in.  Therefore
     the user must exercise 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.

     As an active/passive architecture, 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, the
     active device is kicked out of the MULTIPATH GEOM class and the next in a
     list is selected, the failed I/O reissued and the system proceeds.

     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 label, the device is used to either create a new MULTIPATH
     GEOM, or to attach to the end of the list of devices for an existing
     MULTIPATH GEOM.

     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
     occur and cause the attachment and configuration of any (now) new devices
     to occur, causing the taste event described above.

     This means that this active/passive 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 require manual intervention with camcontrol(8) to cause the
     reattachment of repaired device links.

EXAMPLES
     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
	   ECNTX0LUN000000SER10ac0d01
	   mysys# camcontrol inquiry da2 -S
	   ECNTX0LUN000000SER10ac0d01

     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: adding da0 to Fred/b631385f-c61c-11db-b884-0011116ae789
	   GEOM_MULTIPATH: da0 now active path in Fred
	   GEOM_MULTIPATH: adding da2 to Fred/b631385f-c61c-11db-b884-0011116ae789

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

BUGS
     The gmultipath should allow for a manual method of pairing disks.

     There is currently no way for geom_multipath.ko to distinguish between
     various label instances of the same provider.  That is devices such as
     da0 and da0c can be tasted and instantiated as multiple paths for the
     same device.  Technically, this is correct, but pretty useless.  This
     will be fixed soon (I hope), but to avoid this it is a good idea to
     destroy any label on the disk object prior to labelling it with
     gmultipath.

AUTHOR
     Matthew Jacob ⟨mjacob@FreeBSD.org⟩

BSD			       February 26, 2007			   BSD
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