[dm-devel] device mapper target vs personality, how do you decide to pick one or the other?

[John Utz]

Thankyou for the further clarification. 

My apologies for not being clear about my end goal. Indeed, I do wish to do remapping because I am coding a ZAC simulator.
________________________________________
From: Mike Snitzer [snitzer at redhat.com]
Sent: Friday, July 11, 2014 5:36 PM
To: John Utz
Cc: dm-devel at redhat.com
Subject: Re: device mapper target vs personality, how do you decide to pick one or the other?

On Fri, Jul 11 2014 at  5:56pm -0400,
John Utz <John.Utz at wdc.com> wrote:

> Hi Mike
>
> tnx so much for the prompt response!
>
> sorry i wasnt clear enuf, example of the 'personality code' that i was
> writing about below.
...
> FOR EXAMPLE, here is a little bit of linear.c that contains it:
>
> static struct md_personality linear_personality =
> {
>       .name           = "linear",
>       .level          = LEVEL_LINEAR,
>       .owner          = THIS_MODULE,
>       .make_request   = linear_make_request,
>       .run            = linear_run,
>       .stop           = linear_stop,
>       .status         = linear_status,
>       .hot_add_disk   = linear_add,
>       .size           = linear_size,
> };
>
> static int __init linear_init (void)
> {
>       return register_md_personality (&linear_personality);
> }
>
> static void linear_exit (void)
> {
>       unregister_md_personality (&linear_personality);
> }

The above is MD raid code.  So what you're talking about are the
different MD raid personalities.

> > so, based on this speculation, i *think* i should be using the target
> > construct for the new device mapper module that i am currently
> > coding.
> >
> > am i correct?
>
> Probably.

The target construct is provided by DM.  You haven;t said what you're
looking to do but if it is remapping IO and has nothing to do with raid
then you'd probably want to develop a new DM target.