Date: Wed, 19 Nov 2014 15:07:43 +0100 From: "Patrick M. Hausen" <hausen@punkt.de> To: "freebsd-stable@freebsd.org List" <freebsd-stable@freebsd.org> Subject: 10.1 fresh install and 4k alignment Message-ID: <A2328132-EC43-4E27-8D01-A4D774634598@punkt.de>
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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@seleniumhub:~ # zdb | grep ashift
ashift: 12
But this puzzles me a bit:
root@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
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