help creating new gmirror > 2TB

William Dudley wfdudley at
Thu Aug 31 00:35:15 UTC 2017

I've never heard of AFD, and is having 4K sectors something I have control
of?  Or is tthat something the manufacturer controls?

My drives are: 2 of Seagate 4TB IronWolf NAS SATA 6Gb/s NCQ 64MB Cache
3.5-Inch Internal Hard Drive (ST4000VN008)

Which doesn't have the magic word "AFD" anywhere in the description.

Also, I happen to like the idea of "fixing" the gmirror code to put it's
metadata in the penultimate sector if it discovers GPT
in the last sector, but I suppose I shouldn't get my hopes up.

Bill Dudley

On Wed, Aug 30, 2017 at 5:57 PM, Frank Leonhardt <frank2 at> wrote:

"I'm sure you'll be okay with MBR assuming it's an AFD (4K sector drive)."

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.
> 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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