[gpfsug-discuss] Question about NSD "Devtype" setting, nsddevices file

[Bryan Banister]

Hi all,

We are reviewing some of our configurations and were not sure what to make of the NSD Device Types that GPFS uses and what, if anything, do they change about how GPFS accesses/recovers/manages/etc the underlying storage based on this setting.

The documentation doesn't say much about it other than to consult the /usr/lpp/mmfs/bin/mmdevdiscover command (no man page), which has this section:

# Known disk types currently are:
#
#   powerdisk  - EMC power path disk
#   vpath      - IBM virtual path disk
#   dmm        - Device-Mapper Multipath (DMM)
#   dlmfdrv    - Hitachi dlm
#   hdisk      - AIX hard disk
#   lv         - AIX logical volume.  Historical usage only.
#                Not allowed as a new device to mmcrnsd.
#   gpt        - GPFS partition on Windows disk
#   generic    - Device having no unique failover or multipathing
#                characteristic (predominantly Linux devices).
#   dasd       - DASD device (for Linux on z Systems)

We have our storage under Linux Device-Mapper Multipath control (two device paths to all storage, active/passive) and are accessible under /dev/mapper, but the NSD types are current set to 'generic' not 'dmm'.  This is configured in the /var/mmfs/etc/nsddevices file:

if [[ $osName = Linux ]]
then
  : # Add function to discover disks in the Linux environment.
  ls -l /dev/mpath/ 2>/dev/null | awk '{print "mpath/"$9 " generic"}'
  ls -l /dev/mapper/ 2>/dev/null | awk '{print "mapper/"$9 " generic"}'
  ls -1 /dev/vd* 2>/dev/null | awk -F '/' '{print ""$3 " generic"}'
fi

Can somebody from IBM explain what the correct setting should be and what differences GPFS does with 'generic' vs. 'dmm' vs. others?

Thanks in advance!
-Bryan

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