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[dm-devel] HDS multipathing prioritizer not doing what it should



Hi all,


I think I found a bug in the HDS prioritizer module at http://git.kernel.org/gitweb.cgi?p=linux/storage/multipath/hare/multipath-tools.git;a=blob_plain;f=libmultipath/prioritizers/hds.c;hb=HEAD

In there the following is stated for assigning the priority:


* CONTROLLER ODD and LDEV ODD: PRIORITY 1
* CONTROLLER ODD and LDEV EVEN: PRIORITY 0
* CONTROLLER EVEN and LDEV ODD: PRIORITY 0
* CONTROLLER EVEN and LDEV EVEN: PRIORITY 1

When watching multipathing with debug output one can see that the controllers returned are 1 and 2:

May 08 14:44:00 | sdo: hds prio: VENDOR:  HITACHI
May 08 14:44:00 | sdo: hds prio: PRODUCT: DF600F         
May 08 14:44:00 | sdo: hds prio: SERIAL:  0x0089
May 08 14:44:00 | sdo: hds prio: LDEV:    0x0004
May 08 14:44:00 | sdo: hds prio: CTRL:    1                        <= This is really controller 0
May 08 14:44:00 | sdo: hds prio: PORT:    C
May 08 14:44:00 | sdo: hds prio: CTRL ODD, LDEV EVEN, PRIO 0
May 08 14:44:00 | sdo: hds prio = 0

May 08 14:44:00 | sdk: hds prio: VENDOR:  HITACHI
May 08 14:44:00 | sdk: hds prio: PRODUCT: DF600F         
May 08 14:44:00 | sdk: hds prio: SERIAL:  0x0089
May 08 14:44:00 | sdk: hds prio: LDEV:    0x0004
May 08 14:44:00 | sdk: hds prio: CTRL:    2                       
<= This is really controller 1
May 08 14:44:00 | sdk: hds prio: PORT:    C
May 08 14:44:00 | sdk: hds prio: CTRL EVEN, LDEV EVEN, PRIO 1
May 08 14:44:00 | sdk: hds prio = 1

This looks fine, but afaik HDS starts counting controllers from 0 (so actually I have 0 and 1). So when assigning LUN ownership in the storage, a LUN with an active/passive path will actually always be accessed through the wrong controller. This has a huge performance penalty when the system is under stress, because of the additional overhead generated by this.

To sum this up, the priority is exactly swapped from what it should be:

Lun 0 mapped with ownership on Controller 0 (
CONTROLLER EVEN and LDEV EVEN) will be accessed on Controller 1
Lun 1 mapped with ownership on Controller 1 (
CONTROLLER ODD and LDEV ODD)   will be accessed on Controller 0

I am not quite sure were to fix this. Looks like the code was contributed by Hitachi in 2006. Back then they maybe started the numbering of the controllers with 1. The AMS and the new HUS systems start at 0 though.


If you can tell me how I can help, I am glad to do so.

Thanks,
Christian

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Christian Schausberger, MSc

  Systems Engineer

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