[dm-devel] multipath.conf and polling_interval option seems misleading

[shane bradley]

Yeah the wording I think is the problem. Someone familiar with source 
code or multipathing will read that option differently.

When someone reads the word path, they think of a single path to lun, 
then wait, check the next path to single lun.
They are  not thinking of all paths are checked at same time.

After thinking about it, this would not be good behaviour because it 
could take forever (minutes) before a path is found dead.
I think the problem is just the description of the option 
"polling_interval", it should be made clearer.

--sbradley

Chandra Seetharaman wrote:
> On Fri, 2009-01-16 at 11:03 -0500, shane bradley wrote:
>   
>> After reviewing the code and doing some testing I have noticed that 
>> polling_interval did not work as expected.
>> I had reviewed the description of the option for multipath.conf and it 
>> conflicted with the results that I had got
>> testing device-mapper-multipath on RHEL4/RHEL5.
>>
>> $ cat /usr/share/doc/device-mapper-multipath-0.4.7/multipath.conf.annotated
>> #       # name    : polling_interval
>> #       # scope   : multipathd
>> #       # desc    : interval between two path checks in seconds
>> #       # default : 5
>> #       #
>> #       polling_interval 10
>>
>> ---------
>>
>> The behaviour that I had expected based on the option's description above:
>> check path 1
>> wait polling_interval
>> check path 2
>> wait polling_interval
>> check path 1
>> wait polling_interval
>> check path 2
>> wait polling_interval
>>
>> However after testing the results that I got was(with multipathd -v4):
>> example:
>> check path 1
>> check path 2
>> wait polling_interval
>> check path 1
>> check path 2
>> wait polling_interval
>>
>> ---------
>>
>> The behaviour I seen in RHEL4 and RHEL5 was working as design after
>> reviewing the code and talking to a couple engineers.
>>
>> The problem it seems is how I was reading the description of the option.
>>  From my results in testing and talking with some engineers the 
>> "polling_interval" option actually means:
>>
>> "The interval between checking all possible paths for all multipath paths"
>> ----------
>>
>> 1) Is my assumption correct that "polling_interval" actually means:
>> "The interval between checking all possible paths for all multipath paths"
>>
>> 2) What is a better way to describe the "polling_interval" option?
>>
>> 3) Shouldn't we make it clearer for people who don't that that much 
>> experience with multipathing?
>>     
>
> IMO, the behavior seen is the proper behavior.
>
> If it does as per your interpretation, the seconds between checking of
> the same path will depend on the number of paths to a storage, which may
> not be acceptable.
>
> May be the wording in multipath.conf.annotated should be made clear.
>
>   
>> --sbradley
>>
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>>     
>
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