Getting device-mapper failure notifications

Getting device-mapper failure notifications

[Avishay Traeger]

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Hi all,
I have a system where various configurations are possible - iSCSI/FC,
single paths and multiple.  For multiple I of course use device-mapper.  I
was wondering if it was possible to:
1. use it for cases where I have a single path.
2. create a monitoring process that can take some action (e.g., send a
notification via message queue) in the case of some failure that would
normally cause failover.

Any help would be much appreciated.

Thanks,
Avishay

-- 
*Avishay Traeger, PhD*
*System Architect*

Mobile: +972 54 447 1475
E-mail: avishay@stratoscale.com



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Re: Getting device-mapper failure notifications

[Andy Grover]

On 02/01/2016 11:07 PM, Avishay Traeger wrote:
> Hi all,
> I have a system where various configurations are possible - iSCSI/FC,
> single paths and multiple.  For multiple I of course use device-mapper.
> I was wondering if it was possible to:
> 1. use it for cases where I have a single path.
> 2. create a monitoring process that can take some action (e.g., send a
> notification via message queue) in the case of some failure that would
> normally cause failover.

Does dm-multipath support a single path? I don't know...

but for your #2, dm-multipath publishes a uevent when a path fails, 
maybe the process could look for that? Using libudev?

HTH -- Andy

Re: Getting device-mapper failure notifications

[Avishay Traeger]

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On Wed, Feb 24, 2016 at 7:00 AM, Andy Grover <agrover@redhat.com> wrote:

> On 02/01/2016 11:07 PM, Avishay Traeger wrote:
>
>> Hi all,
>> I have a system where various configurations are possible - iSCSI/FC,
>> single paths and multiple.  For multiple I of course use device-mapper.
>> I was wondering if it was possible to:
>> 1. use it for cases where I have a single path.
>> 2. create a monitoring process that can take some action (e.g., send a
>> notification via message queue) in the case of some failure that would
>> normally cause failover.
>>
>
> Does dm-multipath support a single path? I don't know...
>
> but for your #2, dm-multipath publishes a uevent when a path fails, maybe
> the process could look for that? Using libudev?
>
> HTH -- Andy
>
>
Thanks for pointing me in the right direction Andy!

-- 
*Avishay Traeger, PhD*
*System Architect*

Mobile: +972 54 447 1475
E-mail: avishay@stratoscale.com



Web <http://www.stratoscale.com/> | Blog <http://www.stratoscale.com/blog/>
 | Twitter <https://twitter.com/Stratoscale> | Google+
<https://plus.google.com/u/1/b/108421603458396133912/108421603458396133912/posts>
 | Linkedin <https://www.linkedin.com/company/stratoscale>

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Re: Getting device-mapper failure notifications

[Hannes Reinecke]

On 02/24/2016 01:00 PM, Andy Grover wrote:
> On 02/01/2016 11:07 PM, Avishay Traeger wrote:
>> Hi all,
>> I have a system where various configurations are possible - iSCSI/FC,
>> single paths and multiple.  For multiple I of course use device-mapper.
>> I was wondering if it was possible to:
>> 1. use it for cases where I have a single path.
>> 2. create a monitoring process that can take some action (e.g., send a
>> notification via message queue) in the case of some failure that would
>> normally cause failover.
> 
> Does dm-multipath support a single path? I don't know...
> 
Pshaw.
Of course it does.

(Might be that RH has some magic disallowing that, but certainly there's
nothing in the source which forbids it. In fact I recommend it when
using iSCSI)

So I recommend to always use dm-multipath and listen to uevents.

> but for your #2, dm-multipath publishes a uevent when a path fails,
> maybe the process could look for that? Using libudev?
> 
Alternatively (if you don't want the failover to happen from multipath
itself) you can implement your own device_handler sending out an uevent
and waiting for some magic sysfs attribute to be written to before
completing.

Should be rather trivial.

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: Getting device-mapper failure notifications

[Bryn M. Reeves]

On Wed, Feb 24, 2016 at 06:25:43PM +0800, Hannes Reinecke wrote:
> On 02/24/2016 01:00 PM, Andy Grover wrote:
> > On 02/01/2016 11:07 PM, Avishay Traeger wrote:
> >> Hi all,
> >> I have a system where various configurations are possible - iSCSI/FC,
> >> single paths and multiple.  For multiple I of course use device-mapper.
> >> I was wondering if it was possible to:
> >> 1. use it for cases where I have a single path.
> >> 2. create a monitoring process that can take some action (e.g., send a
> >> notification via message queue) in the case of some failure that would
> >> normally cause failover.
> > 
> > Does dm-multipath support a single path? I don't know...
> > 
> Pshaw.
> Of course it does.
> 
> (Might be that RH has some magic disallowing that, but certainly there's
> nothing in the source which forbids it. In fact I recommend it when
> using iSCSI)

No but find_multipaths is enabled by default so maps are only created
automatically when we see at least two devices with the same WWID.

> So I recommend to always use dm-multipath and listen to uevents.

Ack.

Regards,
Bryn.
 

Re: Getting device-mapper failure notifications

[Mike Snitzer]

On Wed, Feb 24 2016 at  5:25am -0500,
Hannes Reinecke <hare@suse.de> wrote:

> On 02/24/2016 01:00 PM, Andy Grover wrote:
> > On 02/01/2016 11:07 PM, Avishay Traeger wrote:
> >> Hi all,
> >> I have a system where various configurations are possible - iSCSI/FC,
> >> single paths and multiple.  For multiple I of course use device-mapper.
> >> I was wondering if it was possible to:
> >> 1. use it for cases where I have a single path.
> >> 2. create a monitoring process that can take some action (e.g., send a
> >> notification via message queue) in the case of some failure that would
> >> normally cause failover.
> > 
> > Does dm-multipath support a single path? I don't know...
> > 
> Pshaw.
> Of course it does.
> 
> (Might be that RH has some magic disallowing that, but certainly there's
> nothing in the source which forbids it. In fact I recommend it when
> using iSCSI)

No, RH provided dm-multipath doesn't disallow single path multipath.
Not sure why you engage in raising such doubt -- red hat follows
upstream.  period.

multipathd can be configured to look for multiple paths
('find_multipaths yes' in multipath.conf) but there is nothing in the
kernel disallowing the use of dm-multipath with a single path.