>> MD can provide the real block devices.
>>
>> LVM can be a user-space abstraction. All your admin tools would
>> work in terms of an LVM, perhaps defined in /etc/lvm.conf.
>> The kernel filesystem code uses multiple block devices, but you
>> don't have to know that.
>
> MD does provide only one level of LVM functionality (physical
> volume handling) while LVM also provides volume groups and
> logical volumes (the actual block devices).
Yes, I'm well aware of that.
> When using MD to provide physical volumes we would need multiple
> mapping in the kernel (LV -> PE -> PV).
No, the filesystem maps directly to multiple block devices.
You may use md for the block devices if you want RAID,
and the user-space tools could hide that detail.
> In addition using a separate MD layer requires a sepapare
> administration concept. If you're hiding MD from the user you
> could as well merge it into LVM.
The filesystem must be aware of the block devices so that we
can do real-time IO bandwidth allocation like SGI's Irix does.
Since the filesystem must know the underlying structure anyway,
there is no reason to also have an LVM in the kernel. Using the
normal admin tools, you would not notice a difference.
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