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				.\" Copyright (c) 2007 Matthew Jacob
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				.\" $FreeBSD$
				.\"
				.Dd September 8, 2016
				.Dt GMULTIPATH 8
				.Os
				.Sh NAME
				.Nm gmultipath
				.Nd "disk multipath control utility"
				.Sh SYNOPSIS
				.Nm
				.Cm create
				.Op Fl ARv
				.Ar name
				.Ar prov ...
				.Nm
				.Cm label
				.Op Fl ARv
				.Ar name
				.Ar prov ...
				.Nm
				.Cm configure
				.Op Fl APRv
				.Ar name
				.Nm
				.Cm add
				.Op Fl v
				.Ar name prov
				.Nm
				.Cm remove
				.Op Fl v
				.Ar name prov
				.Nm
				.Cm fail
				.Op Fl v
				.Ar name prov
				.Nm
				.Cm restore
				.Op Fl v
				.Ar name prov
				.Nm
				.Cm rotate
				.Op Fl v
				.Ar name
				.Nm
				.Cm prefer
				.Op Fl v
				.Ar name
				.Ar prov
				.Nm
				.Cm getactive
				.Op Fl v
				.Ar name
				.Nm
				.Cm destroy
				.Op Fl v
				.Ar name
				.Nm
				.Cm stop
				.Op Fl v
				.Ar name
				.Nm
				.Cm clear
				.Op Fl v
				.Ar prov ...
				.Nm
				.Cm list
				.Nm
				.Cm status
				.Nm
				.Cm load
				.Nm
				.Cm unload
				.Sh DESCRIPTION
				The
				.Nm
				utility is used for device multipath configuration.
				.Pp
				The multipath device can be configured using two different methods:
				.Dq manual
				or
				.Dq automatic .
				When using the
				.Dq 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 will not be detected automatically.
				The
				.Dq automatic
				method uses on-disk metadata to detect device and all its paths.
				Metadata use the last sector of the underlying disk device and
				include device name and UUID.
				The UUID guarantees uniqueness in a shared storage environment
				but is in general too cumbersome to use.
				The name is what is exported via the device interface.
				.Pp
				The first argument to
				.Nm
				indicates an action to be performed:
				.Bl -tag -width ".Cm destroy"
				.It Cm create
				Create multipath device with
				.Dq 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.
				.Pp
				.Fl A
				option enables Active/Active mode,
				.Fl R
				option enables Active/Read mode, otherwise Active/Passive mode is used
				by default.
				.It Cm label
				Create multipath device with
				.Dq automatic
				method.
				Label the first given provider with on-disk metadata using the specified
				.Ar name .
				The rest of given providers will be retasted to detect these metadata.
				It reliably protects against specifying unrelated providers.
				Providers with no matching metadata detected will not be added to the device.
				.Pp
				.Fl A
				option enables Active/Active mode,
				.Fl R
				option enables Active/Read mode, otherwise Active/Passive mode is used
				by default.
				.It Cm configure
				Configure the given multipath device.
				.Pp
				.Fl A
				option enables Active/Active mode,
				.Fl P
				option enables Active/Passive mode,
				.Fl R
				option enables Active/Read mode.
				.It Cm add
				Add the given provider as a path to the given multipath device.
				Should normally be used only for devices created with
				.Dq manual
				method, unless you know what you are doing (you are sure that it is another
				device path, but tasting its metadata in regular
				.Dq automatic
				way is not possible).
				.It Cm remove
				Remove the given provider as a path from the given multipath device.
				If the last path removed, the multipath device will be destroyed.
				.It Cm fail
				Mark specified provider as a path of the specified multipath device as failed.
				If there are other paths present, new requests will be forwarded there.
				.It Cm restore
				Mark specified provider as a path of the specified multipath device as
				operational, allowing it to handle requests.
				.It Cm rotate
				Change the active provider/path to the next available provider in Active/Passive mode.
				.It Cm prefer
				Change the active provider/path to the specified provider in Active/Passive mode.
				.It Cm getactive
				Get the currently active provider(s)/path(s).
				.It Cm destroy
				Destroy the given multipath device clearing metadata.
				.It Cm stop
				Stop the given multipath device without clearing metadata.
				.It Cm clear
				Clear metadata on the given provider.
				.It Cm list
				See
				.Xr geom 8 .
				.It Cm status
				See
				.Xr geom 8 .
				.It Cm load
				See
				.Xr geom 8 .
				.It Cm unload
				See
				.Xr geom 8 .
				.El
				.Sh SYSCTL VARIABLES
				The following
				.Xr sysctl 8
				variable can be used to control the behavior of the
				.Nm MULTIPATH
				GEOM class.
				.Bl -tag -width indent
				.It Va kern.geom.multipath.debug : No 0
				Debug level of the
				.Nm MULTIPATH
				GEOM class.
				This can be set to 0 (default) or 1 to disable or enable various
				forms of chattiness.
				.It Va kern.geom.multipath.exclusive : No 1
				Open underlying providers exclusively, preventing individual paths access.
				.El
				.Sh EXIT STATUS
				Exit status is 0 on success, and 1 if the command fails.
				.Sh MULTIPATH ARCHITECTURE
				This is a 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
				.Ar indicate
				identity, but in all respects such identity can rarely be considered
				.Ar definitive .
				.Pp
				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 different 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 configuration selection.
				.Pp
				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 at the same time.
				Requests are distributed between paths to equalize load.
				For capable devices it allows the utilisation of the bandwidth available on all paths.
				In Active/Read mode all paths not marked FAIL may handle reads at the same time,
				but unlike in Active/Active mode only one path handles write requests at any
				point in time; closely following the original write request order if the layer
				above needs it for data consistency (not waiting for requisite write completion
				before sending dependent write).
				.Pp
				When new devices are added to the system the
				.Nm MULTIPATH
				GEOM class is given an opportunity to taste these new devices.
				If a new
				device has a
				.Nm MULTIPATH
				on-disk metadata label, the device is either used to create a new
				.Nm MULTIPATH
				GEOM, or added to the list of paths for an existing
				.Nm MULTIPATH
				GEOM.
				.Pp
				It is this mechanism that works reasonably with
				.Xr isp 4
				and
				.Xr mpt 4
				based Fibre Channel disk devices.
				For these devices, when a device disappears
				(due to e.g., 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
				.Nm MULTIPATH
				failure event just described.
				.Pp
				When Fibre Channel events inform either
				.Xr isp 4
				or
				.Xr 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.
				.Pp
				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).
				.Pp
				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
				.Xr camcontrol 8
				to cause the reattachment of repaired device links.
				.Sh EXAMPLES
				The following example shows how to use
				.Xr camcontrol 8
				to find possible multiple path devices and to create a
				.Nm MULTIPATH
				GEOM class for them.
				.Bd -literal -offset indent
				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
				.Ed
				.Pp
				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 storage is qualified to make this judgement.
				.Pp
				You can then use the
				.Nm
				command to label and create a
				.Nm MULTIPATH
				GEOM provider named
				.Ar FRED .
				.Bd -literal -offset indent
				gmultipath label -v FRED /dev/da0 /dev/da2
				disklabel -Brw /dev/multipath/FRED auto
				newfs /dev/multipath/FREDa
				mount /dev/multipath/FREDa /mnt....
				.Ed
				.Pp
				The resultant console output looks something like:
				.Bd -literal -offset indent
				GEOM_MULTIPATH: da0 added to FRED
				GEOM_MULTIPATH: da0 is now active path in FRED
				GEOM_MULTIPATH: da2 added to FRED
				.Ed
				.Pp
				To load the
				.Nm
				module at boot time, add this entry to
				.Pa /boot/loader.conf :
				.Bd -literal -offset ident
				geom_multipath_load="YES"
				.Ed
				.Sh SEE ALSO
				.Xr geom 4 ,
				.Xr isp 4 ,
				.Xr mpt 4 ,
				.Xr loader.conf 5 ,
				.Xr camcontrol 8 ,
				.Xr geom 8 ,
				.Xr mount 8 ,
				.Xr newfs 8 ,
				.Xr sysctl 8
				.Sh HISTORY
				The
				.Nm
				utility first appeared in
				.Fx 7.0
				.Sh AUTHORS
				.An Matthew Jacob Aq Mt mjacob@FreeBSD.org
				.An Alexander Motin Aq Mt mav@FreeBSD.org