approximating freebsd gmirror in dragonflybsd

[Roman Neuhauser]

# alex at alexhornung.com / 2014-07-29 07:52:15 +0100:
> On 2014-07-28 17:05, Roman Neuhauser wrote:
> > # justin at shiningsilence.com / 2014-07-27 17:49:38 -0400:
> >> I've done similar - streaming to a local disk.  There's no way with
> >> just Hammer to have it automatically fail over; you'll have to
> >> manually mark the working disk as the master and throw out the failed
> >> former master.  For something truly automatic, you'd have to go for
> >> hardware RAID.  There's some things possible with software RAID -
> >> vinum or ccd or LVM, but you're wandering into uncharted territory
> >> there.
> > 
> > based on info from alexh re device mapper[0,1], ccd(4) and vinum(8)
> > i'd say i'm out of luck.
> > 
> > [0]
> >   18:52 <@alexh> mirror? it's a return-error shim.
> >   18:52 <@alexh> dmirror has a basic skeleton and some notes on how to
> >                  implement, but that's it
> >   18:53 <@alexh> personally I don't think that sort of raid is the way 
> > to
> >                  go; filesystems are a better place for that sort of 
> > thing
> 
> vinum and cc should work just fine. I'm not sure why you imply that I 
> said otherwise.

sorry i made it sound that way, i meant to attribute only the lvm
assessment to you.

the ccd and vinum mention was with these in mind:

ccd(4) on Disk Mirroring:

     Note that a one-disk ccd is not the same as the original partition.  In
     particular, this means if you have a filesystem on a two-disk mirrored
     ccd and one of the disks fail, you cannot mount and use the remaining
     partition as itself; you have to configure it as a one-disk ccd.  You
     cannot replace a disk in a mirrored ccd partition without first backing
     up the partition, then replacing the disk, then restoring the partition.

vinum(8) in Gotchas:

    vinum drives are UNIX disk partitions and must have the partition
    type vinum.

    The vinum start command will not accept a drive on partition ``c''.
    Partition ``c'' is used by the system to represent the whole disk,
    and must be of type unused.  Clearly there is a conflict here, which
    vinum resolves by not using the ``c'' partition.

so ccd(4) needs maintenance to boot from a degraded partition, and vinum(4)
can't mirror below the slice level.  i want a whole-disk mirror, and i want to
boot from it.

-- 
roman