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[dm-devel] Re: [PATCH 0/3] sysfs representation of stacked devices (dm/md)

On Feb 17, 2006, at 14:42, Alasdair G Kergon wrote:
On Fri, Feb 17, 2006 at 01:00:17PM -0500, Jun'ichi Nomura wrote:
Though md0, dm-0, dm-1 and sd[a-d] contain same LVM2 meta data, LVM2 should pick up md0 as PV, not dm-0, dm-1 and sdXs. mdadm should build md0 from dm-0 and dm-1, not from sdXs. Similar things will happen on 'mount' and 'fsck' if we use file system labels instead of LVM2.

I can't speak for the 'mount' code base, but I don't think it'll make any significant difference to LVM2 - we'd still have to do all the same device scanning as we do now because we have to be aware of md devices defined in on-disk metadata regardless of whether or not the kernel knows about them at the time the command is run.

Aha! This is a very valid reason why we should export partition types from the kernel to userspace: Partitions/devices that appear to have 2 different filesystems/formats. The _kernel_ cannot reliably tell which to use. On the other hand, a properly configured _userspace_ initramfs could use configured partition-type information, a small config file, and a user-configurable detection algorithm to figure out that the device is _actually_ the first segment of an ext3-on-LVM-on-RAID1, instead of a raw ext3, and mount it appropriately. Now, this requires that the admin correctly specify the partition types, but that seems a bit more reliable than depending on the probe-order to get things right.

Kyle Moffett

Unix was not designed to stop people from doing stupid things, because that would also stop them from doing clever things.
  -- Doug Gwyn

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