approximating freebsd gmirror in dragonflybsd

[Justin Sherrill]

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.

On the other hand, you can stream to multiple targets.  A big enough
power surge will kill both disks.  Streaming to a mirrored disk
doesn't give you immediate failover, but streaming to a different
location in addition to streaming to a local disk will make it more
likely your data survives.

A @reboot command for hammer mirror-stream in cron will suffice to
keep it working.

On Sun, Jul 27, 2014 at 12:27 PM, Roman Neuhauser <neuhauser at sigpipe.cz> wrote:
> hello,
>
> i've been using freebsd on RAID1 (2 disks in gmirror[0]) for the past
> ~10 years, and the mirrorring has been wonderful as i never had to
> restore anything from backups, just move the array to a new pair of
> disks[1], with essentially no downtime.  when one of the two disks
> failed, i took the machine down, replaced the failed disk, and booted.
> after some hours of slow-but-steady function, i could replace the other
> old disk.  three times over the years, no sweat at all.
>
> i'd like to replicate this capability with HAMMER, on two 128GB SSDs,
> with 16GB RAM.  i was planning to have the SSDs mirrorred based on [2],
> but the drives show up as daX, and natacontrol ignores them.
>
> i don't want to use hw raid, what are my options?
>
> with freebsd gmirror over da0, da1, if da0 went down, the system would
> automatically boot from da1.  freebsd would degrade the array but there
> would be no maintenance needed to bring the system up when it had to be
> rebooted.
>
> it looks like hammer mirror-stream requires hammer pfs-upgrade before
> the system will boot fully from the slave after the master failed?
>
> what do other people here use for automatic failover?  what do you use
> to make sure the mirror is streaming at all times?
>
> [0] http://mdoc.su/f/gmirror.8
> [1] i'm paranoid, whenever one disk turned sour i threw out both
> [2] http://www.dragonflybsd.org/docs/docs/howtos/howtosoftwareraid/
>
> --
> roman