ZFS root File System

Matthew Dillon dillon at apollo.backplane.com
Fri Feb 27 19:32:29 PST 2009

    My experience with one of our people trying to do the same thing 
    w/ HAMMER... we got it working, but it is not necessarily cleaner.

    I'd rather just boot from a small UFS /boot partition on 'a' (256M
    or 512M), followed by swap on 'b', followed by the big-ass root
    partition on 'd' using your favorite filesystem.

    The boot code already pretty much handles this state of affairs, one only

    (1) To partition it this way.

    (2) Add line to /boot/loader.conf pointing the kernel at the actual root,
	e.g. (in my case):


    (3) Adjust sysctl kern.bootfile in e.g. /etc/sysctl.conf.  Since the
	boot loader thinks the kernel is on / instead of /boot (because
	/boot is the root from the point of view of the bootloader),
	it might set this to "/kernel" instead of "/boot/kernel".  So
	you may have to override it to make crash dumps and name lists
	work properly.

    (4) Add a mount for the little /boot partition in /etc/fstab.

    Trying to create one large root on 'a' puts the default spot for swap
    on 'b' at the end of the disk instead of near the beginning.  The end
    of the disk (closer to the spindle) is a bad place for swap.  Having
    a small /boot partition there instead retains the ordering and puts the
    swap where it is expected to be.

    # df
    Filesystem         1K-blocks     Used     Avail Capacity  Mounted on
    /dev/ad6s1d        193888256  1662976 192225280     1%    /
    /dev/ad6s1a           257998   110896    126464    47%    /boot


    In anycase, if RAID is an issue the loader could always be adjusted to
    look for a boot partition on multiple disks.  One could then have a /boot
    on two independant disks, or even operate it as a soft-raid-mirror.  It
    seems less of an issue these days since someone with that sort of
    requirement who isn't already net-booting can just pop in a SSD for
    booting which will have approximately the same or better MTBF as the
    motherboard electronics.

    The problem we face with HAMMER is related to the boot loader not being
    able to run the UNDO buffer (yet), so it might not be able to find
    the kernel after a crash.  That and the inconvenient place swap ends up


More information about the freebsd-stable mailing list