Vgdisplay output showing Multiple Alternatelink

[PVM_2]

vgdisplay output showing multiple alternate link.Why these many alternatelink.What is the use?...All link identifying same disk or different?..OS HP-UX 11.23 connected to EMC Clariion Storage..

vgdisplay -v vg64
--- Volume groups ---
VG Name /dev/vg64
VG Write Access read/write
VG Status available, exclusive
Max LV 255
Cur LV 1
Open LV 1
Max PV 16
Cur PV 1
Act PV 1
Max PE per PV 1016
VGDA 2
PE Size (Mbytes) 32
Total PE 31
Alloc PE 31
Free PE 0
Total PVG 0
Total Spare PVs 0
Total Spare PVs in use 0

--- Logical volumes ---
LV Name /dev/vg64/lvol1
LV Status available/syncd
LV Size (Mbytes) 992
Current LE 31
Allocated PE 31
Used PV 1


--- Physical volumes ---
PV Name /dev/dsk/c36t1d7
PV Name /dev/dsk/c37t1d7 Alternate Link
PV Name /dev/dsk/c38t1d7 Alternate Link
PV Name /dev/dsk/c39t1d7 Alternate Link
PV Name /dev/dsk/c28t1d7 Alternate Link
PV Name /dev/dsk/c29t1d7 Alternate Link
PV Name /dev/dsk/c30t1d7 Alternate Link
PV Name /dev/dsk/c31t1d7 Alternate Link
PV Status available
Total PE 31
Free PE 0
Autoswitch On
Proactive Polling On

...
PVM

[sangilak]

Hey,


Alternate Links are your different paths to your physical volume. They will ensure redundancy should one or multiple links to your physical volume break...

So in this example your primary path is /dev/dsk/c36t1d7 and the rest are "backup" paths through which data can flow should there be issues with any of the other paths.


Hope that helps,

sangilak

[Manix]

Looks there are seven alternate links coming
for the single EMC LUN ,this is expected behavior .

Are you using EMC powerpath on 11.23 ?

Thanks
Manix

[PVM_2]

Hi Manix,

No, Powerpath is not installed


...
PVM

[Manix · Accepted solution]

Thats fine ..

PowerPath providse load balancing. PV links will not do that.
PowerPath loads its drivers and intercepts I/O. If you have PowerPath I would not recommend to use PV links & vice versa.

http://h30499.www3.hp.com/t5/LVM-and-VxVM/HP-UX-EMC-PowerPath-power-device-or-pvlinks/m-p/5011296#M43426

Thanks
Manix

[PVM_2]

Hi,

Small query..whether these alternatelinks are created automatically when we connect the system to storage..how to create alternatelinks step by step procedure?..

....
PVM

[g3jza]

Hi,
no the links are not created automatically.
They are specified during the creation or modification of the volume group.

For example:

you have 2 paths for the same disk but currently only one path is activated in 'vgdisplay' - c33t1d0:
c33t1d0
c31t1d0

You want to add the second path to the VG:
# vgextend /dev/vg01 /dev/dsk/c31t1d0

Now you should see the alternate link c31t1d0 in your 'vgdisplay' output.

Or when creating the VG from scratch: Just specify both of the paths on the 'vgcreate' command:

for example: #vgcreate /dev/vg01 /dev/dsk/c33t1d0 /dev/dsk/c31t1d0

[Matti_Kurkela]

To expand on g3jza's answer:

When you import an existing volume group using "vgimport -s", the system _will_ automatically generate alternate links if it can reach the LUN(s) by more than one path. As far as I know, this is the only way HP-UX 11.23 or older can auto-detect alternate links.

So it's technically possible to create a VG manually using just one PV link, then export it with "vgexport -s", re-create the /dev/vg directory and the /dev/vg/group file, and then re-import the VG with "vgimport -s". If you have a lot of LUNs and/or paths, this might be an useful procedure in some situations.

MK

[Yogesh M Puranik]

>>>>

Small query..whether these alternatelinks are created automatically when we connect the system to storage..how to create alternatelinks step by step procedure?..

>>>>

___________

Ans:

Here your primary disk(PV) is c36t1d7.When you assign the LUN from storage with different Controller number but same target and disk number then these disks (PV's) will act as a alternate link.

How to add alternate links ?

while creation of VG itself you can define all PV's or later also you can.

#vgcreate vg64 /dev/dsk/c36t1d7 /dev/dsk/c37t1d7
/dev/dsk/c38t1d7
/dev/dsk/c39t1d7
/dev/dsk/c40t1d7
/dev/dsk/c41t1d7
/dev/dsk/c42t1d7

Note (dev/dsk/c36t1d7 will act as a primary disk )

Or

if you have already created VG vg64 with primary disk (pv) /dev/dsk/c36t1d7 then later you can add alternate disks using

#vgextend /dev/dsk/c37t1d7 /dev/dsk/c38t1d7 ...so on


when /dev/dsk/c36t1d7 will fail or removed from VG then next PV will act as a Primary disk.


Hope this help you.