Re: non fs-data and gpt partitioned md

[Ken Gunderson <kgunders@teamcool.net>]

On Thu, 19 Apr 2012 10:38:23 +1000
NeilBrown <neilb@suse.de> wrote:

> On Wed, 18 Apr 2012 18:18:33 -0600 Ken Gunderson
> <kgunders@teamcool.net> wrote:
> 
> > On Tue, 17 Apr 2012 14:40:10 -0600
> > Ken Gunderson <kgunders@teamcool.net> wrote:
> > 
> > Apologies for following my own post but I guess should elaborate...
> > 
> > > Hello:
> > > 
> > > I'm wanting to set up a new md root lvm based configuration and
> > > after reading various docs am confused about how I should be
> > > going about this. My intent is to have mirrored /boot and swap
> > > partitions and a raid10 / partition with LVM.
> > > 
> > > Issue #1: gpt is recommended over mbr based partitioning for new
> > > installs, even on BIOS based systems (presuming these BIOS will
> > > boot gpt, wh/mine do). Auto-assemble is not recommended for new
> > > installations (my understanding is that it is not necessary with
> > > initramfs) so I presume I should be specifying partition type DA.
> > > However, while gdisk et.al. allow for selecting type FD, they do
> > > not offer DA as an option.
> > 
> > Per <http://www.rodsbooks.com/linux-fs-code.html>, Linux ignores
> > partition type codes.  Yet per man mdadm:
> > 
> > "When creating a partition based array,  using  mdadm  with
> > version-1.x metadata, the partition type should be set to 0xDA (non
> > fs-data).  This type selection allows for greater precision since
> > using any other [RAID auto-detect (0xFD) or a GNU/Linux partition
> > (0x83)], might create prob‐ lems in the event of array recovery
> > through a live cdrom."
> > 
> > So which is it?  Does partition type code matter to md/mdadm or not?
> 
> Both.  Neither. 
> 
> md does handle 0xFD partitions a bit differently, but I recommend not
> using that feature.
> Other than that md igores them.  mdadm ingores them completely.
> 
> But other tools - typically installers - might pay some attention to
> them. Using 0xDA discourages such tools from mishandling them.

Thanks for the clarification.  
 
> > > Issue #2: Is there any reason to prefer 1.0 vs. 1.2 metadata? I
> > > can use either grub2 or Syslinux boot loaders.  My understanding
> > > is that Syslinux supports the former while Grub2 supports 1.2.
> > > All other things being equal, I'd prefer to use Syslinux.  Unless
> > > there is some technical reason to favor 1.2 metadata and/or Grub2.
> > > 
> > > So what would be best practices recommended way to proceed here?
> > 
> > The reason I ask is that I bring such a configuration online on
> > Archlinux by following these instructions:
> > 
> > <https://wiki.archlinux.org/index.php/Software_RAID_and_LVM>
> > 
> > But I'll be damned if I can recover from failed drive simulations -
> > at least reliably, as sometimes it works while others not - so I'm
> > just trying to rule out potential variables here.
> > 
> > Thanks-- Ken
> > 
> 
> Any reason for preferring one of 1.0 and 1.2 is out side of md.
> 
> Maybe you want to be able to mount one half of a RAID1
> independently.  You need 1.0 for that.  Maybe you want to ensure that
> never happens.  Then 1.2 is better. Maybe your boot loaded only works
> with one.  Then the choice is clear.

Well, this only pertains to /boot partition, so there's nothing to stop
one from mixing and matching, e.g. Syslinux using 1.0 metadata for
RAID1 /boot and 1.2 metadata for RAID10 swap and root devices, no? Or
is this not advisable?

Thanks-- Ken

-- 
Ken Gunderson <kgunders@teamcool.net>
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