Port of ZFSOnLinux solution for illumos-gate issue #2663 to FreeBSD

[Richard Yao]

On 11/14/2012 12:34 AM, Steven Hartland wrote:
> Useful stuff, you might be interested in the follow:-
>
> Teach ZFS about geom stripe size so zpools are created with optimum ashift
> http://www.freebsd.org/cgi/query-pr.cgi?pr=173115
>
> I've got an updated patch which allows the min ashift to be configured.
>
> It should be noted that the max working ashift appears to be 17
> due the way labels use zio_read_phys with offset.
>
>    Regards
>    Steve

Your patch modifies autodetection while this patch provides a manual
override. They should be able to co-exist.

With that said, the orignal patch permitted ashift=17, but that was
shown to permit pool corruption:

https://github.com/zfsonlinux/zfs/issues/425

As far as I know, ashift=13 is the highest value permitted on both Linux
and FreeBSD. The code can operate with higher values, but I do not
recommend them.

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