help creating new gmirror > 2TB

Warren Block wblock at
Thu Aug 31 01:00:03 UTC 2017

On Wed, 30 Aug 2017, Frank Leonhardt wrote:

> 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.

At some point in the last couple of years, hrs@ produced a working patch 
which did something like that, although I don't remember the details. 
It moved either the GPT backup table or the gmirror metadata.  It was 
turned down as breaking standards.

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