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Re: [dm-devel] Determine partition prefix used for mpath devices



David Huffman wrote:
> Hannes Reinecke wrote:
>> David Huffman wrote:
>>  
>>> In a script, I need to somehow determine what prefix is being used for
>>> partitions as set by UDEV. It seems that the naming convention is in
>>> flux and could change frequently. When using user_friendly_names the
>>> prefix is set using kpartx within udev rules. My problem is I need to
>>> know what the prefix is so my scripts know what device naming convention
>>> to look for.
>>>
>>> Here are the different iterations of naming schemes:
>>>
>>> mpath0p1
>>> mpath0-part1
>>> mpathap1
>>> mpatha-part1
>>>
>>> Is there a way other than grepping through the udev rules to determine
>>> what the prefix is? The partitions may not exist so I cannot just look
>>> to see what they are. I need to know BEFORE I create partitions. I know
>>> this is a strange request, but I am working with a lot of code that was
>>> designed before multipath was available and using the sys naming is not
>>> an option (i.e. /disk/by-id/wwid).
>>>
>>>     
>> I wouldn't rely on the device-mapper table names, as they could be pretty
>> much everything.
>>
>> To handle this situation I've patches up multipath and kpartx to provide
>> UUID prefixes, which normally can't be influenced from userspace.
>> Multipath devices carry the prefix 'mpath-', kpartx generated devices
>> carry the prefix 'part<num>-'.
>> So by checking the prefix you pretty much know which program generated
>> this particular device.
>>
>> Cheers,
>>
>> Hannes
>>   
> Humm.. Maybe I did not explain myself well enough because I do not
> understand your response. I am trying to determine what prefix is
> currently being used for the partitions by kpartx.
> 
> Example:
> If I am on a SLES9 SP4 system there is a udev rule that states to use
> kpartx -a -p -part so that the prefix is "-part". Partitions that are
> created later are named [alias|mpath|uuid]a-part1.
> If I am on a RHEL 5.1 system, there is a udev rule that states to use
> kpartx -a -p p so that the prefix is "p".
> Partitions that are created later are named [alias|uuid|mpath]0p1
> 
> Right now, the only way I can find out what prefix is used for
> partitions is to grep and awk through the udev rule (which is
> unreliable). I am looking for a better way to see what prefix is
> currently being used by kpartx.
> 
Ah. Leads to the question: Why?
What's the whole point of the exercise?

> When you state that you have a patch to provide UUID prefixes, I do not
> understand. If you are not using "user_friendly_names yes" in
> multipath.conf, isn't it already the uuid? How does this change the
> naming of the partitions used? Right now it could be "-part" or "p".
> 
Nothing to do with multipath.

If you do a 'dmsetup info' you'll see something like:
Name:              3600508b40008ddd70000500000250000
State:             ACTIVE
Read Ahead:        1024
Tables present:    LIVE
Open count:        1
Event number:      19
Major, minor:      253, 11
Number of targets: 1
UUID: mpath-3600508b40008ddd70000500000250000

The last line is the UUID I'm talking about.

> Sorry but my limited knowledge of the naming schemes is probably
> confusing the issue. It appears that there are too many players on the
> field when it comes to naming these devices. (UDEV, multipath,
> device-mapper)
> 
Again, what do you need it for?
The whole exercise is already quite cumbersome if
you have only _one_ device-mapper device for a partition;
however, nothing prevents you on calling kpartx (or
some other tools) again with a different prefix
and hence a different name.

So you'll end up with several device-mapper tables
with identical definitions but different names.
Which one will you pick then?

Cheers,

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


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