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

[Kyle Moffett]

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.

Cheers,
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