Mirroring with Fossil and Venti
D1222778749
Agp
#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.
#