freebsd/share/man/man4/geom_fox.4

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.Dd January 2, 2005
.Os
.Dt GEOM_FOX 4
.Sh NAME
.Nm geom_fox
.Nd "GEOM based basic disk multipathing"
.Sh SYNOPSIS
.Cd "kldload geom_fox"
.Sh DESCRIPTION
The intent of the
.Nm 
framework is to provide basic multipathing support to access direct
access devices.
Basic in the above sentence should be read as:
.Nm
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 (typically 2) host bus adapters access 
the same physical disk drive.
.Pp
Without a multipathing driver the
.Fx
kernel would probe the disks multiple times, resulting in the creation of
multiple 
.Pa /dev
entries for the same underlying physical device.
A unique label written in the GEOM label area allows
.Nm 
to detect multiple paths.  Using this information it creates a unique
.Pa da#.fox
device.
.Pp
The 
.Nm 
device is subsequently used by the
.Fx
kernel to access the disks.
Multiple physical access paths ensure that even in case of a path failure the
.Fx
kernel can continue to access the data.
.Pp
The
.Nm 
driver will disallow write operations to the underlying devices once the 
fox device has been opened for writing.
.Sh EXAMPLES
.Pp
.Bl -bullet -compact
.It
.Nm 
needs a label on the disk as follows in order to work properly:
.Bd -literal
"0123456789abcdef0123456789abcdef"
"GEOM::FOX       <--unique--id-->"
.Ed
.Pp
For the unique ID 16 bytes are available.
The 
.Li GEOM::FOX
is the magic to mark a
.Nm
device.
.Pp
The actual labelling is accomplished by
.Bd -literal 
echo "GEOM::FOX       someid" | dd of=/dev/da2 conv=sync
.Ed
.Pp
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.
.Pp
The Node WWN can be obtained from a verbose boot as in for example
.Bd -literal
isp1: Target 1 (Loop 0x1) Port ID 0xe8 (role Target) Arrived
 Port WWN 0x21000004cfc8aca2
 Node WWN 0x20000004cfc8aca2
.Ed
.Pp
This Node WWN would then be used like so:
.Bd -literal 
echo "GEOM::FOX       20000004cfc8aca2" | dd of=/dev/da2 conv=sync
.Ed
.Pp
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.
.Pp
Once the labelling has been performed and assuming the
.Nm
module is loaded the kernel will inform you that it has found a new
.Nm
device with a message similar to
.Bd -literal 
Creating new fox (da2)
fox da2.fox lock 0xfffffc0000fdba20
.Ed
.Pp
.It
To check which physical devices match a given
.Nm
device:
.Bd -literal -offset indent
# geom fox list
Geom name: da2.fox
Providers:
1. Name: da2.fox
   Mediasize: 73407865344 (68G)
   Sectorsize: 512
   Mode: r0w0e0
Consumers:
1. Name: da2
   Mediasize: 73407865856 (68G)
   Sectorsize: 512
   Mode: r0w0e0
2. Name: da6
   Mediasize: 73407865856 (68G)
   Sectorsize: 512
   Mode: r0w0e0
.Ed
.Pp
.It
To check the status of the
.Nm
components:
.Bd -literal
# geom fox status
   Name  Status  Components
da2.fox     N/A  da2
                 da6
.Ed
.El
.Sh SEE ALSO
.Xr GEOM 4 ,
.Xr geom 8
.Sh CAVEATS
The 
.Nm 
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,
.Nm 
is not able to re-initiate the I/O to an alternative physical path.
.Pp
You have to be very sure to provide a unique label for each of the
.Nm
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 standard.
.Sh BUGS
The
.Nm
framework has only seen light testing.
There definitely might be dragons here.
.Pp
The name
.Nm
is completely obscure.
Just remember that any sly fox has multiple exits from its hole.
.Pp
The examples provided are too FibreChannel-centric.
.Sh AUTHORS
.An -nosplit
The
.Nm 
driver was written by 
.An "Poul-Henning Kamp" Aq phk@FreeBSD.org .
This manual page was written by 
.An "Wilko Bulte" Aq wilko@FreeBSD.org .