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

[John Utz]

Bryn and Mike Thankyou very much for your help. I have a dm target that now loads when the appropriate arguments are passed to dmcreate.

tnx!

johnu
________________________________________
From: dm-devel-bounces at redhat.com [dm-devel-bounces at redhat.com] on behalf of John Utz [John.Utz at wdc.com]
Sent: Monday, July 14, 2014 10:15 AM
To: Mike Snitzer
Cc: dm-devel at redhat.com
Subject: Re: [dm-devel] device mapper target vs personality, how do you decide to pick one or the other?

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.

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