Neil Brown wrote:
Maybe the problem here is thinking of md and dm as different things.
Try just not thinking of them at all.
Think about it like this:
The linux kernel support lvm
The linux kernel support multipath
The linux kernel support snapshots
The linux kernel support raid0
The linux kernel support raid1
The linux kernel support raid5
Use the bits that you want, and not the bits that you don't.
dm and md are just two different interface styles to various bits of
this. Neither is clearly better than the other, partly because
different people have different tastes.
Maybe what you really want is for all of these functions to be managed
under the one umbrella application. I think that is was EVMS tried to
do.
I am under the impression that dm is simpler/cleaner than md. That
impression very well may be wrong, but if it is simpler, then that's a
good thing.
One big selling point that 'dm' has is 'dmraid' - a tool that allows
you to use a lot of 'fakeraid' cards. People would like dmraid to
work with raid5 as well, and that is a good goal.
AFAIK, the hardware fakeraid solutions on the market don't support raid5
anyhow ( at least mine doesn't ), so dmraid won't either.
However it doesn't mean that dm needs to get it's own raid5
implementation or that md/raid5 needs to be merged with dm.
It can be achieved by giving md/raid5 the right interfaces so that
metadata can be managed from userspace (and I am nearly there).
Then 'dmraid' (or a similar tool) can use 'dm' interfaces for some
raid levels and 'md' interfaces for others.
Having two sets of interfaces and retrofiting a new interface onto a
system that wasn't designed for it seems likely to bloat the kernel with
complex code. I don't really know if that is the case because I have
not studied the code, but that's the impression I get, and if it's
right, then I'd say it is better to stick with dm rather than retrofit
md. In either case, it seems overly complex to have to deal with both.
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