The fs(3) device can be used to mirror your fossil and venti partitions using several disks. For example, to setup a mirror named /dev/fs/fossil that is a RAID of /dev/sdC0/fossil and /dev/sdD0/fossil you need to

bind -a '#k' /dev
echo mirror fossil /dev/sdC0/fossil /dev/sdD0/fossil >/dev/fs/ctl

Then you can run fossil using /dev/fs/fossil. It is better to setup the mirror before formatting the partition and setting up fossil on it.

If you already have a fossil and want to add another disk to mirror it, you can setup the mirror as said before, with the original fossil device (possibly a partition) being mirrored by the new disk, start your fossil and while you are using it, use dd(1) to copy the fossil device to the mirror device or partition. Since writes go to all devices in the mirror but reads are performed just by the first device (as long as it has no read errors), the copy can be made this way with relative safety. Races are still possible, since dd might read a block from the fossil device, fossil might change it and write it to both devices (original fossil and mirror) and then dd might write the now-stale block to the mirror, so it's safer to make the copy when fossil (and venti, if you mirror it) are idle. This might require booting your file server off another standalone Plan 9 file & cpu server. So, after the dd on an active fossil, cmp(1) the original and mirror devices to be sure that a race didn't hit you while copying. If it did, repeat the procedure or make the copy with all disks idle.

If you are using a standalone fossil system you need to set up devfs to read the configuration from a file located somewhere on the disk. The easiest way to do it (and the least error-prone) is to steal a single-block partition from the ones already on the disk and to put the configuration there, as you would do with the nvram partition. Here's an example:

 # disk/prep /dev/sdC0/plan9
   ...
   swap               20482 1273392     (1252910 sectors, 611.77 MB)
   ...
 >>> d swap
 >>> a fscfg 20482 20483
 >>> a swap 20483 1273392
 >>> w
 >>> q
 #

(note that you may want to create such a partition on each disc that is mirrored with devfs, and even mirror the partition itself, to make sure that if a disk goes away the others will know about the system's disk configuration)

Now we need to copy the devfs configuration to the new partition. First zero it to make sure there's nothing dangling in there:

# dd -if /dev/zero -of /dev/sdC0/fscfg -count 1

Copy the devfs configuration file to a safe place where you can modify it, and prepend 'fsdev:\n' to the beginning of it:

# echo 'fsdev:' > $home/devfs.conf
# cat /dev/fs/ctl >> $home/devfs.conf

Now your devfs.conf file should look something like:

# cat devfs.conf
fsdev:
mirror fossil /dev/sdC0/fossil /dev/sdD0/fossil
#

Copy that to the partition where the configuration will be stored:

# cp devfs.conf /dev/sdC0/fscfg

If the configuration file is longer than 512 bytes you will need to create a bigger partition in which to hold it. Repeat the disk/prep part with as many blocks as you need. Note that devfs.c imposes a hard limit of 1024 bytes per configuration.

Now that the configuration file is in place, add a line to plan9.ini, telling it where it is:

fsconfig=/dev/sdC0/fscfg

You need to specify a particular disk, because devfs isn't started yet, and /dev/fs doesn't exist.

Another alternative is to place the config file into your kernel's /boot. Just add the path to your conf file (eg /usr/bootes/devfs.conf) to the bootdir section at the end of /sys/src/9/pc/pccpuf (or whatever kernel configuration you're using) and recompile (see Compiling kernels). Then in plan9.ini, add:

fsconfig=/boot/devfs.conf

Now configure fossil to search for the disks in /dev/fs/* and you're ready to boot. Here's a sample fossil proto file:

# cat flproto
fsys main config /dev/fs/fossil
fsys main open
fsys main
...
#

Write the config file using fossil/conf to the disk where fossil will search it for:

# cat flproto | fossil/conf -w /dev/fs/fossil

And you should be set.