Index: head/sbin/geom/class/multipath/gmultipath.8
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--- head/sbin/geom/class/multipath/gmultipath.8	(revision 285665)
+++ head/sbin/geom/class/multipath/gmultipath.8	(revision 285666)
@@ -1,375 +1,373 @@
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 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
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 .\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 17, 2015
+.Dd July 18, 2015
 .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 won't be detected automatically.
+Additional device paths also will not be detected automatically.
 The
 .Dq automatic
-method uses on-disk metadata to detect device and all it's paths.
+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 to utilize the bandwidth of 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.
 It allows to closer follow 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
-.Ed
 .Pp
 To load the
 .Nm
-Module at boot time, add the following entry to
-.Pa /boot/loader.conf:
+module at boot time, add this entry to
+.Pa /boot/loader.conf :
 .Bd -literal -offset ident
 geom_multipath_load="YES"
-.Ed
 .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 AUTHORS
 .An Matthew Jacob Aq Mt mjacob@FreeBSD.org
 .An Alexander Motin Aq Mt mav@FreeBSD.org