[dm-devel] Newbie questions

[jon at kollegiegaarden.dk]

On Fri, Nov 28, 2003 at 02:46:49PM +1100, Alec Thomas wrote:
> Hello,
> 
> I have a couple of questions about device-mapper. I've had a look at the
> source and could not see an obvious answer.
> 
> Firstly, is it possible to utilize one device-mapper module from another
> without redirecting to the actual device? For example, if I create a new
> device-mapper module that maps blocks over the network and I want to
> layer dm-crypt on top, am I forced to first map my device, then map
> dm-crypt to this device. It would be extremely convenient to be able to
> "layer" them without this requirement, so I could effectively call
> dm-crypt to do mapping directly from my module.

i dont believe any such structure has been made yet, but i suppose
that if you just call the right functions in the dm-crypt.c module
and pass the right parameters you can make it work.


> My second question is somewhat related. I am thinking about doing a
> network based block device running on device mapper. What is the "best"
> way of communicating with user-space from a device-mapper module? I
> would like to do most of the network stuff in user-space to take
> advantage of existing cluster communication infrastructure, so it would
> need to be fairly high-speed for transfer of blocks to/from user-space.

All blocks are transfered using a bio structure.


> So, to summarize in a less rambling way :)...
> 
> Can device-mapper modules "chain" together without bouncing back through
> device nodes?

you have to do it inside your module


> What is the "best" way of communicating at high volume with user-space
> from a device-mapper module?

all data goes back and forth through struct bio.




JonB