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Re: [dm-devel] Priozier for remote port on fibre channel devices

On 01/11/2013 08:11 AM, Andreas Bleischwitz continental-corporation com wrote:
Hello List,

I'm currently facing a problem with a storage virtualization device
which is located in two different datacenters and shows up as
another path to a configured SAN-LUN.
The virtualization device is from EMC² and is called V-PLEX.

So I have something like this:

         DC A
Host ---VPLEX-DC A---LUN storage A
   |     |
   |     DC B  |
   ------VPLEX-DC B---LUN storage B

The VPLEX device itself is for storage based mirroring and
synchronizes write access to storage A to storage B and the other
way around.

My problem is, that the virtual LUN I get from the VPLEX does not
have any prioritization given by dm-multipath and looks like this:

mpath3 (36000144000000010e06d8f12ec30dfef) dm-9 EMC,Invista
[size=5.0G][features=1 queue_if_no_path][hwhandler=0][rw]
\_ round-robin 0 [prio=1][active]
  \_ 5:0:1:2 sdad 65:208 [active][ready]
  \_ 5:0:4:2 sdak 66:64  [active][ready]
  \_ 5:0:5:2 sdap 66:144 [active][ready]
  \_ 4:0:0:2 sdc  8:32   [active][ready]
  \_ 4:0:1:2 sdh  8:112  [active][ready]
  \_ 4:0:4:2 sdo  8:224  [active][ready]
  \_ 4:0:5:2 sdt  65:48  [active][ready]
  \_ 5:0:0:2 sdy  65:128 [active][ready]

That way, write access is made round-robin and access to the
"remote-devices" - device located in the other DC - creating
duplicate traffic through the
SAN switches as the access has to be routed to the remote VPLEX-B
and it will write to the local VPLEX-A.

I wanted to solve this problem by giving priorities to a path by its
remote-port - which is different for each connection to a SAN
device. Unfortunately
there currently doesn't exist any priorizer and I have to write one
on my own.
To make the definition flexible, I would like to have a file with a
list of port-wwns associated with a priority.

For versions shipped with RHEL5 this could be a simple shell script
which simply looks up the sysfs file structures of a given device.
For RHEL6 this has to be
a binary library. So writing a binary version will make more sense
than maintaining two different solutions.

Well, at least on SLES we're shipping a prioritizer named 'weightedpath' which does exactly this.

However, chances are that the VPLEX is supporting ALUA, so you might want to use the 'alua' prioritizer here.

What is the output of 'sg_inq /dev/sdc' ?


Dr. Hannes Reinecke		      zSeries & Storage
hare suse de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)

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