Can DM-Multipath work on non-SCSI block device

Can DM-Multipath work on non-SCSI block device

[Ahmed Al-Mehdi]

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Hello,

I read the documentation on the dm-multipath home page: htttp://
christophe.varoqui.free.fr/refbook.html , that describes how the userspace
multipath tool detects and maps (scsi) devices.  However, would like to
confirm, does the dm-multipath utilty only work with SCSI devices.  Is
there any option to make it (or fake it) to work with non-SCSI  block
device.

Thank you,
Ahmed.

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Re: Can DM-Multipath work on non-SCSI block device

[Hannes Reinecke]

On 03/14/2013 08:58 AM, Ahmed Al-Mehdi wrote:
> Hello,
>
> I read the documentation on the dm-multipath home page:
> htttp://christophe.varoqui.free.fr/refbook.html
> <http://christophe.varoqui.free.fr/refbook.html> , that describes
> how the userspace multipath tool detects and maps (scsi) devices.
> However, would like to confirm, does the dm-multipath utilty only
> work with SCSI devices.  Is there any option to make it (or fake it)
> to work with non-SCSI  block device.
>
If you can figure out a way on how to get a unique _disk_ id without 
SCSI, sure.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)

Re: Can DM-Multipath work on non-SCSI block device

[Bryn M. Reeves]

On 03/14/2013 09:29 AM, Hannes Reinecke wrote:
> On 03/14/2013 08:58 AM, Ahmed Al-Mehdi wrote:
>> Hello,
>>
>> I read the documentation on the dm-multipath home page:
>> htttp://christophe.varoqui.free.fr/refbook.html
>> <http://christophe.varoqui.free.fr/refbook.html> , that describes
>> how the userspace multipath tool detects and maps (scsi) devices.
>> However, would like to confirm, does the dm-multipath utilty only
>> work with SCSI devices.  Is there any option to make it (or fake it)
>> to work with non-SCSI  block device.
>>
> If you can figure out a way on how to get a unique _disk_ id without
> SCSI, sure.

E.g. cciss devices are supported using the cciss_id program - that's a 
good example of a very "non-scsi" device type working with multipath.

As well as the WWID problem you'll need to come up with some scheme for 
faking unique HCIL values for the devices since multipath userspace 
cares about that. The cciss approach is to force lun and channel to '0' 
for all devices and to use the CCISS controller (cN) and disk (dN) 
values for the host and id (aka target) values.

See the functions in discovery.c with a cciss_ prefix for more examples.

Regards,
Bryn.

Re: Can DM-Multipath work on non-SCSI block device

[Hannes Reinecke]

On 03/14/2013 11:41 AM, Bryn M. Reeves wrote:
> On 03/14/2013 09:29 AM, Hannes Reinecke wrote:
>> On 03/14/2013 08:58 AM, Ahmed Al-Mehdi wrote:
>>> Hello,
>>>
>>> I read the documentation on the dm-multipath home page:
>>> htttp://christophe.varoqui.free.fr/refbook.html
>>> <http://christophe.varoqui.free.fr/refbook.html> , that describes
>>> how the userspace multipath tool detects and maps (scsi) devices.
>>> However, would like to confirm, does the dm-multipath utilty only
>>> work with SCSI devices.  Is there any option to make it (or fake it)
>>> to work with non-SCSI  block device.
>>>
>> If you can figure out a way on how to get a unique _disk_ id without
>> SCSI, sure.
>
> E.g. cciss devices are supported using the cciss_id program - that's
> a good example of a very "non-scsi" device type working with multipath.
>
> As well as the WWID problem you'll need to come up with some scheme
> for faking unique HCIL values for the devices since multipath
> userspace cares about that. The cciss approach is to force lun and
> channel to '0' for all devices and to use the CCISS controller (cN)
> and disk (dN) values for the host and id (aka target) values.
>
> See the functions in discovery.c with a cciss_ prefix for more
> examples.
>
Or the dasd variant of those.

However, the HCIL variable is mostly cosmetic.
The disk id is the true issue; without it you cannot do multipathing.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)