On Tue, Oct 8, 2013 at 1:17 PM, Nathan Whitehorn <nwhiteh...@freebsd.org>wrote:

> 4. What is this gnop stuff for?

Can't comment on the rest, but gnop is required to create 4K-aligned vdevs
where the minimum block size is 4K (aka ashift=12).  Without this, ZFS
relies on the underlying disk driver providing the correct information, and
most don't.  Also, if you don't do this, and create a vdev using 512B
sectors, the ashift will be set to 9, and replacing the drive down the line
with 4K Advanced Format drive will drop your drive performance in the

Thus, to future-proof your pool, you need to:
  - set the ashift of the pool to 12 (4 KB)
  - align the disk/partition on 4 KB boundaries (starting partition at 1 MB
works well)

Until our ZFS gains the ability to set a minimum ashift for the pool, or to
set the ashift at vdev creation, or all drive manufacturers write perfect
firmware, than we need to fake it with gnop.

Freddie Cash
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