multipath: expected results of configuring "no_path_retry"?

multipath: expected results of configuring "no_path_retry"?

[Diedrich Ehlerding]

I have a question concerning the expected behavior of dm_multipath 
connected to an EMC Clariion array. Different versions of multipath 
give different results when I disconnect all pathes to the storage. 

I have one server (SuSE SLES10 SP2, 2.6.16.60-0.3, multipath-tools 
0.4.7-34.43) and another one (SLES 11,  2.6.27.19-5-default, 0.4.8-
40.1). Both versions recognize the Clariion without further setting in 
multipath.conf. The settings for the Clariion in multipath -t seem to 
be the same (especially queue_if_no_path + no_path_retry 60)

Trying to setup a software mirror on top, i.e. trying to mirror the 
data into a second box, I observed the follwoing diffenrent behavior 
between these two versions: 

- on the SLES10 machine, disconnecting the disk in one Clariion results 
in IO error, the mirror breaks up, and the use IO continues on the 
surviving part of the mirror. This is the behavior which I want, and 
which I expected, and this was my understanding of "no_path_retry" - 
retry some times, then terminate IO.  

- on the SLES11 machine, the same attempt makes IO hang infinitely. The 
messages display that all pathes fail, and then "Entering recovery 
mode: max_retries=60" - but then, nothing happens; IOs hang.

I tried to modify no_path_retry=1 in multipath.conf, and according to 
multipath -t, this setting is accepted - but again, the messages say 
"Retry=60"; i.e. this setting seems to be hardcoded into dm_emc.

It is, however, possible to set "features 0" in multipath.conf; now it 
seems to work as desired - i.e. with this setting, IO doesnt hang, but 
is terminated with an IO error when I disconnect the Lun. 

My question is: which behaviour SHOULD I expect - is the SLES10 
behavior correct (retry 60 times, and then create an error); or is the 
SLES11 behavor correct (wait infinitely), if I use the default settings 
for a Clariion?

TIA
D. 

Re: multipath: expected results of configuring "no_path_retry"?

[Hannes Reinecke]

Diedrich Ehlerding wrote:
> I have a question concerning the expected behavior of dm_multipath 
> connected to an EMC Clariion array. Different versions of multipath 
> give different results when I disconnect all pathes to the storage. 
> 
> I have one server (SuSE SLES10 SP2, 2.6.16.60-0.3, multipath-tools 
> 0.4.7-34.43) and another one (SLES 11,  2.6.27.19-5-default, 0.4.8-
> 40.1). Both versions recognize the Clariion without further setting in 
> multipath.conf. The settings for the Clariion in multipath -t seem to 
> be the same (especially queue_if_no_path + no_path_retry 60)
> 
> Trying to setup a software mirror on top, i.e. trying to mirror the 
> data into a second box, I observed the follwoing diffenrent behavior 
> between these two versions: 
> 
> - on the SLES10 machine, disconnecting the disk in one Clariion results 
> in IO error, the mirror breaks up, and the use IO continues on the 
> surviving part of the mirror. This is the behavior which I want, and 
> which I expected, and this was my understanding of "no_path_retry" - 
> retry some times, then terminate IO.  
> 
> - on the SLES11 machine, the same attempt makes IO hang infinitely. The 
> messages display that all pathes fail, and then "Entering recovery 
> mode: max_retries=60" - but then, nothing happens; IOs hang.
> 
Yes, I know. You are triggering Novell bugzilla #485281

Patch is already available from my multipath repository

git://git.kernel.org/pub/scm/linux/kernel/git/hare/multipath-tools.git
branch sles11

Will be included in the next maintenance update.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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