10.1 fresh install and 4k alignment

[Mark Martinec]

Patrick M. Hausen wrote:
> Hi, all,
> 
> I just installed a new machine with 10.1-RELEASE using the memstick 
> installer
> and chosing ZFS mirror root as the disk layout.
> 
> I did check the "4k" option, watched the installer do the necessary 
> gnop dance
> and the pool seems to be OK:
> 
> root at seleniumhub:~ # zdb | grep ashift
>             ashift: 12
> 
> But this puzzles me a bit:
> 
> root at seleniumhub:~ # gpart list ada0
>    Mediasize: 524288 (512K)
>    Sectorsize: 512
> ...
> Providers:
> 1. Name: ada0p1
>    Mediasize: 524288 (512K)
>    Sectorsize: 512
> ...
>    end: 1057
>    start: 34
> ...
> 3. Name: ada0p3
>    Mediasize: 465747565056 (434G)
>    Sectorsize: 512
> ...
>    type: freebsd-zfs
> ...
>    end: 976773134
>    start: 67109922
> 
> None of the start sector numbers is a multiple of 8, neither are the 
> end sectors
> a multiple of 8 minus 1.
> 
> So the pool uses a 4k block size but it starts on an odd multiple of
> 2k on the platter
> - do I see this correctly?
> 
> Isn't it absolutely necessary that the simulated 4k blocks are laid
> out so that the first
> is made from 512 byte sectors 0-7, the second from 8-15 and so on?
> 
> Then why does the installer start the first partition at 512 byte
> sector 34? While I'm at
> it, this magic number is everywhere in the older documentation, e.g.
> for manually
> installing FreeBSD 8 with gptzfsboot - where does that 34 come from? 
> It's not
> a power of 2 nor is it one of the historical CHS magic numbers that 
> would mean
> a cylinder boundary or similar. Size of the GPT partition table itself?
> 
> Is this a bug in the installer? Will I still have to layout the disks
> manually if I want 4k
> alignment? What's a good offset for the first partition in this case?
> Anything bigger
> than 34 that's a multiple of 8 - 40 or 64?
> 
> Thanks in advance
> Patrick


Reported on 2014-10-10, but was apparently forgotten/ignored:

   "GPT partitions not 4k aligned by 10.1-RC1 installer"
   
https://lists.freebsd.org/pipermail/freebsd-stable/2014-October/080509.html


Mark