Volume Manager (DMP)/EMC Power Path interoperability

Below is the link to EMC Support Matrix and can be accessed at the EMC web site at the following location. See the Veritas Volume Manager section in this document. 

 

 

https://elabnavigator.emc.com/vault/pdf/EMC_PowerPath.pdf

As of version 4.1 (to include all subsequent versions of Volume Manager/Storage Foundation), Volume Manager contains a feature known as TPD (Third Party Drivers).  PowerPath will claim the multiple paths of the LUNs on the array and DMP will manage the single path to the PowerPath pseudo device.

The way EMC PowerPath will work is that it will claim the multiple paths of the LUNs on the array as shown from the command below (showing the hardware path and WWN or c#t#d#s#).
 

Example is given in Solaris but conceptually applies to all PowerPath supported Unix environments.

# powermt display dev=all

Pseudo name=emcpower5a
Symmetrix ID=000190301018
Logical device ID=02CD
state=alive; policy=SymmOpt; priority=0; queued-IOs=0
==============================================================================
---------------- Host ---------------   - Stor -   -- I/O Path -  -- Stats ---
###  HW Path                I/O Paths    Interf.   Mode    State  Q-IOs Errors
==============================================================================
3073 pci@1f,2000/fibre-channel@1/fp@0,0 c1t5006048AD5F0D690d609s0 FA  1dA   active  alive      0      0
3073 pci@1f,2000/fibre-channel@1/fp@0,0 c1t5006048AD5F0D69Fd431s0 FA 16dA   active  alive      0      0

Pseudo name=emcpower4a
Symmetrix ID=000190301018
Logical device ID=02CE
state=alive; policy=SymmOpt; priority=0; queued-IOs=0
==============================================================================
---------------- Host ---------------   - Stor -   -- I/O Path -  -- Stats ---
###  HW Path                I/O Paths    Interf.   Mode    State  Q-IOs Errors
==============================================================================
3073 pci@1f,2000/fibre-channel@1/fp@0,0 c1t5006048AD5F0D690d610s0 FA  1dA   active  alive      0      0
3073 pci@1f,2000/fibre-channel@1/fp@0,0 c1t5006048AD5F0D69Fd432s0 FA 16dA   active  alive      0      0

Pseudo name=emcpower3a
Symmetrix ID=000190301018
Logical device ID=02CF
state=alive; policy=SymmOpt; priority=0; queued-IOs=0
==============================================================================
---------------- Host ---------------   - Stor -   -- I/O Path -  -- Stats ---
###  HW Path                I/O Paths    Interf.   Mode    State  Q-IOs Errors
==============================================================================
3073 pci@1f,2000/fibre-channel@1/fp@0,0 c1t5006048AD5F0D690d611s0 FA  1dA   active  alive      0      0
3073 pci@1f,2000/fibre-channel@1/fp@0,0 c1t5006048AD5F0D69Fd433s0 FA 16dA   active  alive      0      0
...


From above, PowerPath will create a single "pseudo device" for all paths leading to the same physical LUN and that device is presented to the OS.


(/dev/dsk and /dev/rdsk entries are created by PowerPath)


#echo|format
...
     19. emcpower3a <EMC-SYMMETRIX-5772 cyl 3278 alt 2 hd 15 sec 128>
         /pseudo/emcp@3
     20. emcpower4a <EMC-SYMMETRIX-5772 cyl 3278 alt 2 hd 15 sec 128>
         /pseudo/emcp@4
     21. emcpower5a <EMC-SYMMETRIX-5772 cyl 3278 alt 2 hd 15 sec 128>
         /pseudo/emcp@5
...


The single path of the pseudo devices is claimed by Volume Manager's DMP component and a DMP node is created.



# vxdisk list
DEVICE       TYPE            DISK         GROUP        STATUS
...
emcpower3s2  auto:cdsdisk    -            -            online
emcpower4s2  auto:cdsdisk    -            -            online
emcpower5s2  auto:cdsdisk    -            -            online
...


The result is that Volume Manager's DMP manages the single path to the PowerPath pseudo device and PowerPath manages the multiple paths to the LUN.


# vxdisk list emcpower5s2
Device:    emcpower5s2
...
Multipathing information:
numpaths:   1   <<<<<<<<<<<<<<<<<<<<<<
emcpower5c      state=enabled
...


When a path failure to the array takes place, EMC PowerPath will perform the appropriate dynamic multi-pathing operation.  When the device and/or all paths to a device have failed, PowerPath will mark the pseudo device as failed (SCSI transport error is logged to syslog).  Volume Manager will then detect that the single path to the pseudo device has failed and since it is the last path to the device (the only path), Volume Manager will then mark the device as failed.