Native multipathing with legacy DSF

[Eric SAUBIGNAC]

Bonsoir,

I have read somewhere (release notes, white paper, ?) that there is a kernel parameter that controls activation of native multipathing over legacy DSF ...

Was it a dream or something real ? Could someone point a document speaking about this feature ?

Thanks in advance

Eric

[James R. Ferguson]

Hi Eric:

There are a number of documents in the 11iv3 Core HP-UX section of the new and improved HP documentation website.

Among then, look for:

HP-UX 11i v3 Mass Storage Device Naming

HP-UX 11i v3 Mass Storage I/O Performance Improvements

HP-UX 11i v3 Native Multi-Pathing for Mass Storage (September 2009)'

Regards!

...JRF...

[Matti_Kurkela]

http://bizsupport2.austin.hp.com/bc/docs/support/SupportManual/c01916036/c01916036.pdf

See chapter titled "Impact of Native Multi-Pathing on LVM", starting at the bottom of page 4.

In a nutshell: the use of native multipathing is not controlled by a kernel parameter, but by the scsimgr command.

For example, to (non-persistently) disable multipathing on legacy DSFs associated with /dev/(r)disk/disk5:

scsimgr set_attr -D /dev/rdisk/disk5 -a leg_mpath_enable=false

To make the setting globally (for all legacy DSFs on the system) just omit the "-D " part.

To make the setting persist through reboots, use "scsimgr save_attr" instead of "scsimgr set_attr".

MK

[Eric SAUBIGNAC]

@James : I was expecting a more precise link pointing straight to the answer. Maybe I am too lazy to RTFM ;-)

@Matti : thanks for this precise link. But as I understand your answer, it speaks of multipathing in a general way, and the scsimgr attribut leg_mpath_enable applies to agile DSF, not to legacy DSF. Right ?

Maybe I should ask my question in a different way :

>>> do we have multipathing features, at least failover, when using legacy DSF ?

I found in the document pointed by Matti, at the top of page 5 :

--------
HP-UX 11i v3 provides native multi-pathing functionality, .../... This is the default behavior in the case of active/active devices, irrespective of the naming model used.
--------

The "irrespective" word let me think that the answer is yes. Right ?

>>> Is there a way, kernel parameter, klm module, scsimgr parameter, or anything else, to control native multipathing with legacy DSF (I really don't speak of agile DSF) ?


Eric

[Eric SAUBIGNAC]

@Matti

Oups ... as usually english seems to be a problem for me. Your answer has pointed exactly what I was wainting for. Don't know why, but I missunderstood your post ;-(

Also found in "HP Integrity VM Accelerated Virtual I/O (AVIO) Overview" @ http://bizsupport2.austin.hp.com/bc/docs/support/SupportManual/c02018682/c02018682.pdf , bottom of page 19 :

-------------
Multipathing Tunables
HP-UX 11iv3 supports multi-pathing even using legacy device names. Multi-pathing using legacy DSFs
is controlled by the scsimgr tunable â leg_mpath_enableâ which is set by default. This tunable should
not be altered in the host.
-------------

It explicitly answers the question.

So Matti, please post, so that I can deserve right points

Eric

[Matti_Kurkela · Accepted solution]

I almost sent a copy of the appropriate parts of the document I originally linked, with the key sentences -->> explicitly highlighted <<--.

In a nutshell: when the leg_mpath_enable is true (= the default setting), the legacy DSFs will behave just like they do with EMC PowerPath on HP-UX 11.23 or older: use any one legacy DSF and you'll get a load-balanced connection over all available paths to that LUN.

This also means that the link-switching at the LVM level (with pvchange -s or -S) effectively does nothing at all. An attempt to disable a particular legacy path with "pvchange -a n" is also a no-op. You should use scsimgr instead for these actions.

If you set leg_mpath_enable to false, the legacy DSFs will emulate the older HP-UX standard behaviour: each legacy DSF is associated with one (and only one) particular path. Path switching will happen at the LVM level.

However, you should still remember to use scsimgr instead of pvchange to disable paths if necessary, because some things (e.g. the HP-UX 11.31 online diagnostics) might still be using the agile DSFs.

MK

[Eric SAUBIGNAC]

Thanks Matti ;-)