crash/README

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# xfscrash #  crash testing setup for XFS
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*** disclaimers ***

    work-in-progress, buyer-beware, your-mileage-may-vary, this-is-a-hack

*** what xfscrash does ***

    xfscrash allows realistic testing of XFS log recovery and XFS check/repair
    by generating log activity on an XFS partition, then rebooting the machine
    at a random point. When the machine comes back up, xfscrash is restarted
    and then tests either the log recovery or xfs_repair on the dirtied
    filesystem. All going well the process continues.

*** getting ready for crash testing ***

    Most filesystems (ext2 included) can't withstand having the machine
    they're running on rebooted while they're active. So the crash test
    machine needs to have all filesystems other than the test FS mounted
    read-only so they won't get trashed when the machine reboots.

*** mouting FSes read-only ***

    Following is a recipe for making a redhat linux (6.2) machine with a single
    ext2 FS mounted on root able to be booted read-only. Your Mileage May
    Vary - don't try this on an important machine.

    The idea is to move anything that needs to be r/w into the /initrd_init
    directory, replacing the moved directories with links to the moved ones.
    That way the /initrd_init directory may be copied to a ramdisk, and
    mounted over /initrd on the root FS which never gets remounted r/w.

        # go to single user
        init 1

        # make a mount point for the ramdisk
        mkdir /initrd

        # link across to the /initrd_init directory for when 
        # the ramdisk isn't mounted
        ln -s /initrd_init/dev .
        ln -s /initrd_init/etc .
        ln -s /initrd_init/proc .
        ln -s /initrd_init/sbin .
        ln -s /initrd_init/tmp .
        ln -s /initrd_init/var .

        # make the /initrd_init directory
        mkdir /initrd_init
        cd /initrd_init


        # move /dev
        mv /dev .
        ln -s /dev /initrd/dev

        # move /etc
        mv /etc .
        ln -s /etc /initrd/etc

        # make proc mount
        mkdir proc 

        # move /tmp
        mkdir tmp
        rm -rf /tmp
        ln -s /tmp /initrd/tmp

        # link /sbin 
        ln -s /sbin .

        # setup a tree for parts of /var
        mkdir var var/cache var/lock var/lock/console var/lock/subsys
        mkdir var/log var/preserve var/run 

        touch /var/run/utmp /var/log/utmp /var/log/wtmp 

        # move parts of /var
        rm -rf /var/cache /var/lock /var/log /var/preserve /var/run
        ln -s /initrd/var/cache /var/cache
        ln -s /initrd/var/lock /var/lock
        ln -s /initrd/var/log /var/log
        ln -s /initrd/var/preserve /var/preserve
        ln -s /initrd/var/run /var/run

        # make a mount for /var/shm
        mkdir var/shm 
        ln -s /var/shm /initrd/var/shm

        # move /var/spool
        mkdir var/spool
        mkdir var/spool/mail var/spool/anacron var/spool/at var/spool/lpd
        mkdir var/spool/rwho var/spool/mqueue var/spool/cron
        rm -rf /var/spool
        ln -s /var/spool /initrd/var/spool

        # move /var/tmp
        mkdir var/tmp
        rm -rf /var/tmp
        ln -s /var/tmp /initrd/var/tmp

        # trim /dev - too many inodes here - remove anything you don't need
        # (small ramdisk has a small number of inodes)
        rm -rf /initrd/dev/<....>   

    All going well, all the directories you've made should link through
    /initrd and into /initrd_init, and the machine should come back up
    if you restart it.

    You want to keep the contents of /initrd_init to a minimum because
    this stuff has to fit into the ramdisk.

*** getting the ramdisk going ***

    See the rc.sysinit file for some details of what to do to get the
    ro-root/ramdisk up and running.

    Once everything is going, the root FS should never be remounted to
    r/w on boot and should be in r/o mode when the machine comes up.
    
    All going well, any open files have been redirected through the
    symlinks onto the ramdisk, so you should be able to remount the
    root FS to r/w and then remount it back to r/o.
    
    Since there's no r/w filesystems mounted, it should be ok to 
    reboot the machine with 'reboot -fn' and everything should come
    back without dirty filesystems and without having to fsck.
    
*** starting xfscrash ***

    The simplest way to restart xfscrash on reboot is to start it
    in the background from rc.local. The script logs to /dev/tty1,
    /dev/console & a logfile by default, so the output should be
    easy to find.
    
    Link the xfscrash directory off an NFS mounted FS so you can make
    changes while the machine is rebooting and so you can touch the
    'stop' and 'start' control files.

    To configure the system, change the parameters in the configuration
    section of the 'xfscrash' script.
    
    To start the system, touch the 'start' control file and then either
    reboot or manually run the 'xfscrash' script.
    
    To stop the system, touch the 'stop' control file and wait for the
    next cycle to start when the control file will be checked and
    the test terminated.