freebsd-skq/sbin/geom/class/multipath/gmultipath.8

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.Dd April 18, 2012
.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.
The
.Dq 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.
.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 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).
.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 used to either create a new
.Nm MULTIPATH
GEOM, or been added 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 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
.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
.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 AUTHOR
.An Matthew Jacob Aq mjacob@FreeBSD.org
.An Alexander Motin Aq mav@FreeBSD.org