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GMULTIPATH(8)           FreeBSD 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 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 detected 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 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.

     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

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

     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 unrelated providers.  Providers with no
                  matching metadata detected will not be added to the device.

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

     configure    Configure the given multipath device.

                  -A option enables Active/Active mode, -P option enables
                  Active/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
                  device.  If the last path removed, the multipath device will
                  be destroyed.

     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.

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

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

     getactive    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
     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
     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.

     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
     requisite 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
     occur 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
     storage 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 <>

FreeBSD 11.0-PRERELEASE         April 18, 2012         FreeBSD 11.0-PRERELEASE


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