device mapper target vs personality, how do you decide to pick one or the other?

device mapper target vs personality, how do you decide to pick one or the other?

[John Utz]

Hi;

having stared at the code associated with these 2 constructs for a while, i cant come up with a clear reason why one would choose one over the other.

having said that, it seems that the personality code is ancestral and is used for the personalities that can be compiled into device mapper and the target code is for newer stuff that is not expected to be compiled into device mapper.

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?

tnx!

johnu

Re: device mapper target vs personality, how do you decide to pick one or the other?

[Mike Snitzer]

On Fri, Jul 11 2014 at  5:28pm -0400,
John Utz <John.Utz@wdc.com> wrote:

> Hi;
> 
> having stared at the code associated with these 2 constructs for a while, i cant come up with a clear reason why one would choose one over the other.
> 
> having said that, it seems that the personality code is ancestral and
> is used for the personalities that can be compiled into device mapper
> and the target code is for newer stuff that is not expected to be
> compiled into device mapper.

Not sure I know what you mean by "personality code" in DM.

> 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.

Re: device mapper target vs personality, how do you decide to pick one or the other?

[John Utz]

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.

(and apologies in advance for my outlook web access MUA's lack of appropriate citing technology)
________________________________________
From: Mike Snitzer [snitzer@redhat.com]
Sent: Friday, July 11, 2014 2:51 PM
To: John Utz
Cc: dm-devel@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  5:28pm -0400,
John Utz <John.Utz@wdc.com> wrote:

> Hi;
>
> having stared at the code associated with these 2 constructs for a while, i cant come up with a clear reason why one would choose one over the other.
>
> having said that, it seems that the personality code is ancestral and
> is used for the personalities that can be compiled into device mapper
> and the target code is for newer stuff that is not expected to be
> compiled into device mapper.

Not sure I know what you mean by "personality code" in DM.

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);
}



> 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.

Re: device mapper target vs personality, how do you decide to pick one or the other?

[Mike Snitzer]

On Fri, Jul 11 2014 at  5:56pm -0400,
John Utz <John.Utz@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.

Re: 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@redhat.com]
Sent: Friday, July 11, 2014 5:36 PM
To: John Utz
Cc: dm-devel@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@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.

Re: 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@redhat.com [dm-devel-bounces@redhat.com] on behalf of John Utz [John.Utz@wdc.com]
Sent: Monday, July 14, 2014 10:15 AM
To: Mike Snitzer
Cc: dm-devel@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@redhat.com]
Sent: Friday, July 11, 2014 5:36 PM
To: John Utz
Cc: dm-devel@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@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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Re: device mapper target vs personality, how do you decide to pick one or the other?

[Bryn M. Reeves]

On Fri, Jul 11, 2014 at 09:28:51PM +0000, John Utz wrote:
> having said that, it seems that the personality code is ancestral and is used for the personalities that can be compiled into device mapper and the target code is for newer stuff that is not expected to be compiled into device mapper.

Not really; it's just a bit confusing as two different but closely
related subsystems live in drivers/md. One is 'md', the Linux
multi-devices subsystem. This is the traditional Linux software RAID
subsystem and offers 'personalities' for different RAID levels and other
mapping types (RAID1-6, JBOD/linear, multipath w/on-disk metadata).

The other is 'dm'; the device-mapper. Using dm offers more flexibility
in specifying virtual devices via the table abstraction (which lets you
glue several targets together a bit like the MD linear personality).
Today dm has targets for linear, zero, error, mirror, snapshot (old),
thin provisioning, caching etc. and also exposes the MD RAID
personalities as dm targets (dm-raid.c; not to be confused with the
userspace dmraid utility).

> 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.

For dm it's always a target: you'd only create a new personality if you
wanted to extend MD's available RAID levels or types.

Regards,
Bryn.

Re: device mapper target vs personality, how do you decide to pick one or the other?

[John Utz]

Bryn;

Thankyou so very much for this answer!

I absolutely failed to understand that these where 2 different subsystems in the same directory, it makes vastly more sense now.

________________________________________
From: dm-devel-bounces@redhat.com [dm-devel-bounces@redhat.com] on behalf of Bryn M. Reeves [bmr@redhat.com]
Sent: Monday, July 14, 2014 4:25 AM
To: device-mapper development
Subject: Re: [dm-devel] device mapper target vs personality, how do you decide to pick one or the other?

On Fri, Jul 11, 2014 at 09:28:51PM +0000, John Utz wrote:
> having said that, it seems that the personality code is ancestral and is used for the personalities that can be compiled into device mapper and the target code is for newer stuff that is not expected to be compiled into device mapper.

Not really; it's just a bit confusing as two different but closely
related subsystems live in drivers/md. One is 'md', the Linux
multi-devices subsystem. This is the traditional Linux software RAID
subsystem and offers 'personalities' for different RAID levels and other
mapping types (RAID1-6, JBOD/linear, multipath w/on-disk metadata).

The other is 'dm'; the device-mapper. Using dm offers more flexibility
in specifying virtual devices via the table abstraction (which lets you
glue several targets together a bit like the MD linear personality).
Today dm has targets for linear, zero, error, mirror, snapshot (old),
thin provisioning, caching etc. and also exposes the MD RAID
personalities as dm targets (dm-raid.c; not to be confused with the
userspace dmraid utility).

> 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.

For dm it's always a target: you'd only create a new personality if you
wanted to extend MD's available RAID levels or types.

Regards,
Bryn.

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