gmirror on 7B4

Clifton Royston cliftonr at
Thu Jan 3 11:25:28 PST 2008

On Wed, Jan 02, 2008 at 08:47:43AM -0800, Chris H. wrote:
> Quoting John Nielsen <lists at>:
> >I'm not sure I remember everything from earlier in this thread so I 
> >don't know if it's relevant, BUT you can't boot from a gstripe volume 
> >(or from a gconcat one AFAIK). Inferring from your fstab example 
> >below it doesn't sound like you intend to but I just wanted to be 
> >sure.
> Are you sure? I read that using gmirror requires /kernel to be located
> in the /boot slice and everything else (all other slices) can be mirrored
> safely. But in all my reading (man pages, FBSD handbook, asstd articles)
> I haven't seen anything indicating booting wasn't possible from a gstripe
> volume.

  Your current idea is backwards; you can boot from entirely mirrored
drives (i.e. RAID1) and I've been doing it since 5.3, but AFAIK it is
impossible to boot from a striped drive and I suspect will remain so
for a long time.

  One way to visualize this is to recognize that because the gmirror
information is stored at the very end of the lower-level GEOM object,
each of the raw drives in the mirrored set appears to be an perfectly
normal drive when reading it from its beginning; thus it is possible to
simply read it as a normal device during the earlier stages of boot
until GEOM and gmirror loads.  With striping, however, the logical
content is spread out across multiple drives, so any one drive you try
to boot from has only 1/Nth of the relevant sectors.

  Does this help?

  -- Clifton

    Clifton Royston  --  cliftonr at / cliftonr at
       President  - I and I Computing *
 Custom programming, network design, systems and network consulting services

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