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33 .Nd "GEOM based basic disk multipathing"
35 .Cd "kldload geom_fox"
39 framework is to provide basic multipathing support to access direct
41 Basic in the above sentence should be read as:
43 only provides path failover functionality, not load balancing over
44 the available paths etc.
45 Using suitable hardware like SCSI or FibreChannel disks it is
46 possible to have multiple (typically 2) host bus adapters access
47 the same physical disk drive.
49 Without a multipathing driver the
51 kernel would probe the disks multiple times, resulting in the creation of
54 entries for the same underlying physical device.
55 A unique label written in the GEOM label area allows
57 to detect multiple paths. Using this information it creates a unique
63 device is subsequently used by the
65 kernel to access the disks.
66 Multiple physical access paths ensure that even in case of a path failure the
68 kernel can continue to access the data.
72 driver will disallow write operations to the underlying devices once the
73 fox device has been opened for writing.
79 needs a label on the disk as follows in order to work properly:
81 "0123456789abcdef0123456789abcdef"
82 "GEOM::FOX <--unique--id-->"
85 For the unique ID 16 bytes are available.
88 is the magic to mark a
92 The actual labelling is accomplished by
94 echo "GEOM::FOX someid" | dd of=/dev/da2 conv=sync
97 For FibreChannel devices it is suggested to use the Node World Wide
98 Name (Node WWN) as this is guaranteed by the FibreChannel standard to
100 The use of the Port WWN not recommended as each port of a given
101 device has a different WWN, thereby confusing things.
103 The Node WWN can be obtained from a verbose boot as in for example
105 isp1: Target 1 (Loop 0x1) Port ID 0xe8 (role Target) Arrived
106 Port WWN 0x21000004cfc8aca2
107 Node WWN 0x20000004cfc8aca2
110 This Node WWN would then be used like so:
112 echo "GEOM::FOX 20000004cfc8aca2" | dd of=/dev/da2 conv=sync
115 For non-FibreChannel devices you could for example use the serial
116 number of the device.
117 Regardless of what you use, make sure the label is unique.
119 Once the labelling has been performed and assuming the
121 module is loaded the kernel will inform you that it has found a new
123 device with a message similar to
125 Creating new fox (da2)
126 fox da2.fox lock 0xfffffc0000fdba20
130 To check which physical devices match a given
133 .Bd -literal -offset indent
138 Mediasize: 73407865344 (68G)
143 Mediasize: 73407865856 (68G)
147 Mediasize: 73407865856 (68G)
153 To check the status of the
158 Name Status Components
169 driver depends on the underlying hardware drivers to do the right thing in case
171 If for example a hardware driver continues to retry forever,
173 is not able to re-initiate the I/O to an alternative physical path.
175 You have to be very sure to provide a unique label for each of the
178 Safety belts are not provided.
179 For FibreChannel devices it is suggested to use the Port WWN of the device.
180 The World Wide Name is guaranteed to be worldwide unique per the
181 FibreChannel standard.
185 framework has only seen light testing.
186 There definitely might be dragons here.
190 is completely obscure.
191 Just remember that any sly fox has multiple exits from its hole.
193 The examples provided are too FibreChannel-centric.
198 driver was written by
199 .An "Poul-Henning Kamp" Aq phk@FreeBSD.org .
200 This manual page was written by
201 .An "Wilko Bulte" Aq wilko@FreeBSD.org .