202 lines
5.5 KiB
Groff
202 lines
5.5 KiB
Groff
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.\"
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.\" Copyright (c) 2006 Wilko Bulte
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd January 2, 2005
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.Os
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.Dt GEOM_FOX 4
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.Sh NAME
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.Nm geom_fox
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.Nd "GEOM based basic disk multipathing"
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.Sh SYNOPSIS
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.Cd "kldload geom_fox"
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.Sh DESCRIPTION
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The intent of the
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.Nm
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framework is to provide basic multipathing support to access direct
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access devices.
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Basic in the above sentence should be read as:
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.Nm
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only provides path failover functionality, not load balancing over
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the available paths etc.
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Using suitable hardware like SCSI or FibreChannel disks it is
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possible to have multiple (typically 2) host bus adapters access
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the same physical disk drive.
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.Pp
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Without a multipathing driver the
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.Fx
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kernel would probe the disks multiple times, resulting in the creation of
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multiple
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.Pa /dev
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entries for the same underlying physical device.
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A unique label written in the GEOM label area allows
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.Nm
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to detect multiple paths. Using this information it creates a unique
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.Pa da#.fox
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device.
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.Pp
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The
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.Nm
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device is subsequently used by the
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.Fx
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kernel to access the disks.
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Multiple physical access paths ensure that even in case of a path failure the
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.Fx
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kernel can continue to access the data.
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.Pp
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The
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.Nm
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driver will disallow write operations to the underlying devices once the
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fox device has been opened for writing.
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.Sh EXAMPLES
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.Pp
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.Bl -bullet -compact
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.It
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.Nm
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needs a label on the disk as follows in order to work properly:
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.Bd -literal
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"0123456789abcdef0123456789abcdef"
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"GEOM::FOX <--unique--id-->"
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.Ed
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.Pp
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For the unique ID 16 bytes are available.
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The
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.Li GEOM::FOX
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is the magic to mark a
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.Nm
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device.
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.Pp
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The actual labelling is accomplished by
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.Bd -literal
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echo "GEOM::FOX someid" | dd of=/dev/da2 conv=sync
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.Ed
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.Pp
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For FibreChannel devices it is suggested to use the Node World Wide
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Name (Node WWN) as this is guaranteed by the FibreChannel standard to
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be worldwide unique.
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The use of the Port WWN not recommended as each port of a given
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device has a different WWN, thereby confusing things.
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.Pp
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The Node WWN can be obtained from a verbose boot as in for example
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.Bd -literal
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isp1: Target 1 (Loop 0x1) Port ID 0xe8 (role Target) Arrived
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Port WWN 0x21000004cfc8aca2
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Node WWN 0x20000004cfc8aca2
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.Ed
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.Pp
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This Node WWN would then be used like so:
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.Bd -literal
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echo "GEOM::FOX 20000004cfc8aca2" | dd of=/dev/da2 conv=sync
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.Ed
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.Pp
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For non-FibreChannel devices you could for example use the serial
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number of the device.
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Regardless of what you use, make sure the label is unique.
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.Pp
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Once the labelling has been performed and assuming the
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.Nm
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module is loaded the kernel will inform you that it has found a new
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.Nm
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device with a message similar to
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.Bd -literal
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Creating new fox (da2)
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fox da2.fox lock 0xfffffc0000fdba20
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.Ed
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.Pp
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.It
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To check which physical devices match a given
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.Nm
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device:
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.Bd -literal -offset indent
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# geom fox list
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Geom name: da2.fox
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Providers:
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1. Name: da2.fox
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Mediasize: 73407865344 (68G)
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Sectorsize: 512
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Mode: r0w0e0
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Consumers:
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1. Name: da2
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Mediasize: 73407865856 (68G)
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Sectorsize: 512
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Mode: r0w0e0
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2. Name: da6
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Mediasize: 73407865856 (68G)
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Sectorsize: 512
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Mode: r0w0e0
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.Ed
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.Pp
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.It
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To check the status of the
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.Nm
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components:
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.Bd -literal
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# geom fox status
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Name Status Components
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da2.fox N/A da2
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da6
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.Ed
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.El
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.Sh SEE ALSO
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.Xr GEOM 4 ,
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.Xr geom 8
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.Sh CAVEATS
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The
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.Nm
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driver depends on the underlying hardware drivers to do the right thing in case
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of a path failure.
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If for example a hardware driver continues to retry forever,
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.Nm
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is not able to re-initiate the I/O to an alternative physical path.
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.Pp
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You have to be very sure to provide a unique label for each of the
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.Nm
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devices.
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Safety belts are not provided.
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For FibreChannel devices it is suggested to use the Port WWN of the device.
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The World Wide Name is guaranteed to be worldwide unique per the
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FibreChannel standard.
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.Sh BUGS
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The
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.Nm
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framework has only seen light testing.
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There definitely might be dragons here.
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.Pp
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The name
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.Nm
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is completely obscure.
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Just remember that any sly fox has multiple exits from its hole.
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.Pp
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The examples provided are too FibreChannel-centric.
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.Sh AUTHORS
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.An -nosplit
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The
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.Nm
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driver was written by
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.An "Poul-Henning Kamp" Aq phk@FreeBSD.org .
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This manual page was written by
|
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.An "Wilko Bulte" Aq wilko@FreeBSD.org .
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