Copyright (c) 2006 Wilko Bulte All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other ...
NAMEgeom_fox - GEOM based basic disk multipathing
SYNOPSISTo compile this driver into the kernel, place the following line in your kernel configuration file:
Alternatively, to load the driver as a module at boot time, place the following line in loader.conf5:
DESCRIPTIONThe intent of the ifconfig framework is to provide basic multipathing support to access direct access devices. Basic in the above sentence should be read as: ifconfig 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 Fx 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 ifconfig to detect multiple paths. Using this information it creates a unique da#.fox device.
The ifconfig 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.
The ifconfig driver will disallow write operations to the underlying devices once the fox device has been opened for writing.
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 ifconfig 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 ifconfig module is loaded the kernel will inform you that it has found a new ifconfig 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 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 status Name Status Components da2.fox N/A da2 da6
SEE ALSOGEOM(4), geom(8), gmultipath(8)
AUTHORSAn -nosplit The ifconfig 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 .
CAVEATSThe ifconfig 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, ifconfig 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 ifconfig 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.
BUGSThe ifconfig framework has only seen light testing. There definitely might be dragons here.
The name ifconfig is completely obscure. Just remember that any sly fox has multiple exits from its hole.
The examples provided are too FibreChannel-centric.