10.1 fresh install and 4k alignment

[Aidan Scheller]

Hi Mark,

Did you submit a PR for this?  I'm not able to find anything related in
Bugzilla and am wondering we should do so.

Thanks,

Aidan

On Wed, Nov 19, 2014 at 8:32 AM, Mark Martinec <Mark.Martinec+freebsd at ijs.si
> wrote:

> 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
>
>
>
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