geom_fox man page on FreeBSD

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GEOM_FOX(4)		 BSD Kernel Interfaces Manual		   GEOM_FOX(4)

     geom_fox — GEOM based basic disk multipathing

     To compile this driver into the kernel, place the following line in your
     kernel configuration file:

	   options GEOM_FOX

     Alternatively, to load the driver as a module at boot time, place the
     following line in loader.conf(5):


     The intent of the geom_fox framework is to provide basic multipathing
     support to access direct access devices.  Basic in the above sentence
     should be read as: geom_fox only provides path failover functionality,
     not load balancing over the available paths etc.  Using suitable hardware
     like SCSI or FibreChannel disks it is possible to have multiple (typi‐
     cally 2) host bus adapters access the same physical disk drive.

     Without a multipathing driver the FreeBSD kernel would probe the disks
     multiple times, resulting in the creation of multiple /dev entries for
     the same underlying physical device.  A unique label written in the GEOM
     label area allows geom_fox to detect multiple paths.  Using this informa‐
     tion it creates a unique device.

     The geom_fox device is subsequently used by the FreeBSD kernel to access
     the disks.	 Multiple physical access paths ensure that even in case of a
     path failure the FreeBSD kernel can continue to access the data.

     The geom_fox driver will disallow write operations to the underlying
     devices once the fox device has been opened for writing.

     ·	 geom_fox needs a label on the disk as follows in order to work prop‐

	 "GEOM::FOX	  <--unique--id-->"

	 For the unique ID 16 bytes are available.  The “GEOM::FOX” is the
	 magic to mark a geom_fox device.

	 The actual labelling is accomplished by

	 echo "GEOM::FOX       someid" | dd of=/dev/da2 conv=sync

	 For FibreChannel devices it is suggested to use the Node World Wide
	 Name (Node WWN) as this is guaranteed by the FibreChannel standard to
	 be worldwide unique.  The use of the Port WWN not recommended as each
	 port of a given device has a different WWN, thereby confusing things.

	 The Node WWN can be obtained from a verbose boot as in for example

	 isp1: Target 1 (Loop 0x1) Port ID 0xe8 (role Target) Arrived
	  Port WWN 0x21000004cfc8aca2
	  Node WWN 0x20000004cfc8aca2

	 This Node WWN would then be used like so:

	 echo "GEOM::FOX       20000004cfc8aca2" | dd of=/dev/da2 conv=sync

	 For non-FibreChannel devices you could for example use the serial
	 number of the device.	Regardless of what you use, make sure the
	 label is unique.

	 Once the labelling has been performed and assuming the geom_fox mod‐
	 ule is loaded the kernel will inform you that it has found a new
	 geom_fox device with a message similar to

	 Creating new fox (da2)
	 fox lock 0xfffffc0000fdba20

     ·	 To check which physical devices match a given geom_fox device:

	       # geom fox list
	       Geom name:
	       1. Name:
		  Mediasize: 73407865344 (68G)
		  Sectorsize: 512
		  Mode: r0w0e0
	       1. Name: da2
		  Mediasize: 73407865856 (68G)
		  Sectorsize: 512
		  Mode: r0w0e0
	       2. Name: da6
		  Mediasize: 73407865856 (68G)
		  Sectorsize: 512
		  Mode: r0w0e0

     ·	 To check the status of the geom_fox components:

	 # geom fox status
	    Name  Status  Components     N/A  da2

     GEOM(4), geom(8), gmultipath(8)

     The geom_fox driver depends on the underlying hardware drivers to do the
     right thing in case of a path failure.  If for example a hardware driver
     continues to retry forever, geom_fox is not able to re-initiate the I/O
     to an alternative physical path.

     You have to be very sure to provide a unique label for each of the
     geom_fox devices.	Safety belts are not provided.	For FibreChannel
     devices it is suggested to use the Port WWN of the device.	 The World
     Wide Name is guaranteed to be worldwide unique per the FibreChannel stan‐

     The geom_fox driver was written by Poul-Henning Kamp ⟨⟩.
     This manual page was written by Wilko Bulte ⟨⟩.

     The geom_fox framework has only seen light testing.  There definitely
     might be dragons here.

     The name geom_fox is completely obscure.  Just remember that any sly fox
     has multiple exits from its hole.

     The examples provided are too FibreChannel-centric.

BSD				January 2, 2005				   BSD

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