GEOM_FOX(4) MidnightBSD Kernel Interfaces Manual GEOM_FOX(4)

NAME

geom_fox — GEOM based basic disk multipathing

SYNOPSIS

kldload geom_fox

DESCRIPTION

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 (typically 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 information it creates a unique da#.fox 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.

EXAMPLES

geom_fox needs a label on the disk as follows in order to work properly:

"0123456789abcdef0123456789abcdef"
"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 module 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 da2.fox lock 0xfffffc0000fdba20

To check which physical devices match a given geom_fox device:

# 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

To check the status of the geom_fox components:

# geom fox status
Name Status Components
da2.fox N/A da2
da6

SEE ALSO

GEOM(4), geom(8)

CAVEATS

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 standard.

BUGS

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.

AUTHORS

The geom_fox driver was written by Poul-Henning Kamp 〈phk@FreeBSD.org〉. This manual page was written by Wilko Bulte 〈wilko@FreeBSD.org〉.

MidnightBSD 0.3 January 2, 2005 MidnightBSD 0.3