help creating new gmirror > 2TB

Frank Leonhardt frank2 at
Wed Aug 30 21:57:40 UTC 2017

On 29/08/2017 21:12, William Dudley wrote:
> Hi,
> I want to create a simple mirror > 2TB on a FreeBSD 10.3 system.
> I have 2 identical 4TB disks.
> The examples in freebsd handbook "geom-mirror" pages show creation of a 2TB
> mirror using
> MBR partitioning, and that has an upper limit of 2TB.
> Some documentation says not to use GPT partitioning with gmirror because
> both store their information in the last sector on the disk.
> I'm not expert enough to be able to solve this myself.
> How do I create a gmirror of 4TB size?
> I want to partition it into 4 slices after I create it, but think I can use
> gpart to do that.
> Note: I'm not interested in using zfs unless there's no way to do this with
> gmirror.
> I read too many zfs failure stories on this mailing list to be comfortable
> with zfs.

I still get a bit worried about this, but I'm 99.9% sure you'll be okay 
with MBR assuming it's an AFD (4K sector drive). The bodge/workaround 
works. If it's SAS instead of SATA, all bets are off.

Although I use ZFS a lot, I still prefer geom mirror for twin-disk 
systems. I feel a lot more comfortable booting from it in the event of a 
failure. ZFS has its good points, but so does UFS.

Trying to get geom mirror to work with GPT as it stands just leads to 
pain. I've taken a look at the code with a view to fixing this is no one 
else does, but UFS is so un-cool in most circles and I don't fancy doing 
it alone in case I zap someone's data. it doesn't look that tricky to 
move the metadata somewhere else, and by checking for a GPT you can 
select between the old/new block. It's unexpected interactions I'm 
worried about.

I think we're safe with MBR until we pass 16Tb.

Regards, Frank.

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