Using GTP and glabel for ZFS arrays

Charles Sprickman spork at
Thu Jul 22 04:20:09 UTC 2010

On Wed, 21 Jul 2010, Adam Vande More wrote:

> On Wed, Jul 21, 2010 at 10:05 PM, Dan Langille <dan at> wrote:
>> Why '-b 34'?  Randi pointed me to
>> where it explains what
>> the first 33 LBA are used for.  It's not for us to use here.
>> Where SOMEVALUE is the number of blocks to use.  I plan not to use all the
>> available blocks but leave a few hundred MB free at the end. That'll allow
>> for the variance in HDD size.
>> Any suggestions/comments?  Is there any advantage to using the -l option on
>> 'gpart add' instead of the glabel above?
> You'll want to make sure your partitions are aligned, discussion here(says
> 4k drives, but info pertinent to all):

>From that thread:

(longer explanation)

I'm not really understanding the alignment issue myself on a few levels:

-Does it only affect the new drives with 4K blocks?
-If it does not, is it generally good to start your first partition at 1MB 
in?  How exactly does doing this "fix" the alignment issue?

> My understanding is that you weren't booting from zfs, just using it as an
> data file system.  In that case, you'd want to use "gpart add -b 512 ..."
> or some other multiple of 16.  Even 1024 would be a good safe number.  Also
> GPT creates partitions not slices.  Your resulting partitions with be
> labeled something like ad0p1, ad0p2, etc.

I assume the same can be applied if you do boot from zfs; you'd still 
create the "freebsd-boot" partition starting at 34, but your next 
partition (be it swap or zfs) would start either 512 or 1024 sectors in?



> -- 
> Adam Vande More
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