[Bug 203180] bsdinstall 4K sector option could/should be persisted in /boot/loader.conf

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=203180

            Bug ID: 203180
           Summary: bsdinstall 4K sector option could/should be persisted
                    in /boot/loader.conf
           Product: Base System
           Version: 10.2-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: misc
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: oliver at openbrackets.net

I wouldn't really classify this as a "bug", but really an improvement to the
bsdinstall "experimental" zfs on root feature => sane defaults.

If you choose "4K Sectors" during a zfs-on-root install in 10.2 RELEASE
bsdinstall correctly sets vfs.zfs.min_auto_ashift=12 and creates the initial
zroot pool vdev using 4K minimum sector size. In my case this was a 2-disk
mirror. 

Further vdevs which are created, in my case with this command, adding a second
mirror to the zroot pool:

zpool add zroot mirror ada2 ada3

This results in the following sector sizes:

root at zfsroot:~ # zdb -l /dev/ada0p3 | grep ashift
        ashift: 12
root at zfsroot:~ # zdb -l /dev/ada1p3 | grep ashift
        ashift: 12
root at zfsroot:~ # zdb -l /dev/ada2 | grep ashift
        ashift: 9
root at zfsroot:~ # zdb -l /dev/ada3 | grep ashift
        ashift: 9

ie, fine (following the installer option) on initial vdev, and back to the OS
default for the next vdev. 

Knowing this I could easily set the sysctl before adding the mirror. My humble
suggestion is that this might be a nice-to-have "sane default" set by
bsdinstall. 

Given the sysadmin has chosen 4K sectors for the initial it is "reasonable" (?)
to assume that this is a modern'ish machine and 4K sectors are better for the
hardware or desired for other reasons. Further, future vdevs created are
probably subject to the same context. 

Could bsdinstall just add the sysctl setting for 4K settings to
/boot/loader.conf if that option is chose during install? After the initial
reboot, the sysadmin would then naturally create vdevs of the same sector size
as chosen at bootup or can change the setting back to 512b (ashift=9). 

It might be worth communicating this loader.conf adjustment to the sysadmin at
an appropriate point.

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