gmirror, gpart and MBR vs GPT in the Handbook

Julien Cigar jcigar at ulb.ac.be
Tue Dec 3 17:37:12 UTC 2013


On Tue, Dec 03, 2013 at 09:27:12AM -0700, Warren Block wrote:
> On Tue, 3 Dec 2013, Julien Cigar wrote:
> 
> > On Mon, Dec 02, 2013 at 08:56:13AM -0700, Warren Block wrote:
> >> On Mon, 2 Dec 2013, Julien Cigar wrote:
> >>
> >>> If you want to use GPT with gmirror you may want to mirror each
> >>> partition instead of the whole disk.
> >>>
> >>> For example on my box I have the following: https://dpaste.de/Rb3S
> >>
> >> There are a couple of potential problems with that.  The big one is when
> >> a disk fails and is replaced.  If you're not careful, the rebuild of all
> >> those mirrored partitions will start at the same time.  Head contention
> >> will bring that to a near-standstill.  It also puts a heavy load on the
> >> drive that still works.  Hopefully it is not the same model and age as
> >> the one that failed, or its "warranty timer" may also be close to
> >> expiring.
> >
> > That's true, I turned off autosynchronization of stale components to
> > avoid this kind of bad scenario ..
> >
> >>
> >> A less-serious problem is that only the partitions are mirrored.  That
> >> leaves out metadata like the partition tables and bootcode, but those
> >> typically do not change very often and might not be a problem.  The
> >> admin has to remember to manually install such things on a new
> >> replacement disk, though.
> >
> > It's just a matter of gpart backup / gpart restore, right ?
> 
> That will get the partition tables but not the bootcode or PMBR.  A 
> failed disk could then leave a mirror which has all the data but is 
> unbootable.

ok, so I've to be sure that:

$> gpart bootcode -b /boot/pmbr -p gptboot -i 1 adaX 

is executed for each disk involved in the mirror, right ..?

> 
> At some point, we'll have to address the conflict between gmirror and 
> GPT.  ZFS mirrors can replace gmirror in some situations, but not all.
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