good/best practices for gmirror and gjournal on a pair of disks?

[George Hartzell]

I've been running many of my systems for some time now using gmirror
on a pair of identical disks, as described by Ralf at:

  http://people.freebsd.org/~rse/mirror/

Each disk has single slice that covers almost all of the disk.  These
slices are combined into the gmirror device (gm0), which is then
carved up by bsdlabel into gm0a (/), gm0b (swap), gm0d (/var), gm0e
(/tmp), and gm0f (/usr).

My latest machine is using Seagate 1TB disks so I thought I should add
gjournal to the mix to avoid ugly fsck's if/when the machine doesn't
shut down cleanly.  I ended up just creating a gm0f.journal and using
it for /usr, which basically seems to be working.

I'm left with a couple of questions though:

  - I've read in the gjournal man page that when it is "... configured
    on top of gmirror(8) or graid3(8) providers, it also keeps them in
    a consistent state..."  I've been trying to figure out if this
    simply falls out of how gjournal works or if there's explicity
    collusion with gmirror/graid3 but can't come up with a
    satisfactory explanation.  Can someone walk me through it?

    Since I'm only gjournal'ing a portion of the underlying gmirror
    device I assume that I don't get this benefit?

  - I've also read in the gjournal man page "... that sync(2) and
    fsync(2) system calls do not work as expected anymore."  Does this
    invalidate any of the assumptions made by various database
    packages such as postgresql, sqlite, berkeley db, etc.... about
    if/when/whether their data is safely on the disk?

  - What's the cleanest gjournal adaptation of rse's
    two-disk-mirror-everything setup that would be able to avoid
    tedious gmirror sync's.  The best I've come up with is to do two
    slices per disk, combine the slices into a pair of gmirror
    devices, bsdlabel the first into gm0a (/), gm0b (swap), gm0d
    (/var) and gm0e (/tmp) and bsdlabel the second into a gm1f which
    gets a gjournal device.

    Alternatively, would it work and/or make sense to give each disk a
    single slice, combine them into a gmirror, put a gjournal on top
    of that, then use bsdlabel to slice it up into partitions?

Is anyone using gjournal and gmirror for all of the system on a pair
of disks in some other configuration?

Thanks,

g.