Regarding alternate links

[Vivek Bhatia]

Hi Experts,

We have a Volume Groups in which we have disks in the sequence below in vgsan02.

PV Name /dev/dsk/c45t0d4
PV Name /dev/dsk/c47t0d4 Alternate Link
PV Name /dev/dsk/c46t0d4 Alternate Link
PV Name /dev/dsk/c48t0d4 Alternate Link

PV Status available
PV Name /dev/dsk/c45t0d3
PV Name /dev/dsk/c47t0d3 Alternate Link
PV Name /dev/dsk/c46t0d3 Alternate Link
PV Name /dev/dsk/c48t0d3 Alternate Link

Now SAN team want us to change the sequence to:

/dev/rdsk/c45t0d4 -Primary
/dev/rdsk/c46t0d4 -Alt
/dev/rdsk/c48t0d4 -Alt
/dev/rdsk/c47t0d4 -Alt


/dev/rdsk/c48t0d3 -Primary
/dev/rdsk/c47t0d3 -Alt
/dev/rdsk/c45t0d3 -Alt
/dev/rdsk/c46t0d3 -Alt


Could you please advice, Is it of any use or not?

* HPUX: 11.11
* Powerpath is not present
* above mentioned are Clariion luns

Thanks in Advance,
Vivek Bhatia

[Duncan Edmonstone]

Probably - presumably the 2nd LUN is active on the other Clariion controller and they want to balance IOs between controllers.

What would be interesting would be to see if c45 and c48 go out of the same FC card on your host - you should be able to figure that out from ioscan.

HTH

Duncan

[Sunny123_1]

Hi

To change your alternate link as primary you can use vgreduce and vgextend to reorganize Alternate links.

vgreduce /dev/vgsan02 /dev/rdsk/c48t0d3
vgextend /dev/vgsan02 /dev/rdsk/c48t0d3

or

try single command
pvchange -s /dev/dsk/c48t0d3
this will set the /dev/dsk/c48t0d3 as the primary disk


Regards
Sunny

[Vivek Bhatia]

Hello All,

Thanks for your Reply but my question is still unanswered.

1. Will changing the Sequence help reducing the affect in case of a disk failure?

2.Can HPUX 11.11 take care itself?

Thanks
Vivek Bhatia

[Ganesan R]

Hi,

>>Could you please advice, Is it of any use or not?<<

Yes. If c45 is primary link for both LUN's than all the i/o will go through single controller which is c45.

If you change the order as per SAN team request, i/o for the first LUN goes through c45 and second LUN goes through c48. Because LVM access the disk always through primary controller till primary link is unavailable.

This way you can share the load.

[Ganesan R]

Hi again,

>>1. Will changing the Sequence help reducing the affect in case of a disk failure?<<

No.Links are no way related to disk failure. If disk fails means all the links are unavailable. It is upto storage RAID level to maintain the disk availablity.


2.Can HPUX 11.11 take care itself?

HP-UX will take care only the load balancing and switching between the links.

[Bill Hassell]

> 1. Will changing the Sequence help reducing the affect in case of a disk failure?

An alternate link is just that -- another way to talk to the disk array. The only failure that alternate links handle is a broken fibre or a bad port (very unusual in most systems). The other purpose for alternate links is to provide additional paths to run multiple I/Os at the same time. This only works if you have load distribution software such as PowerPath. LVM has very limited load distribution capability in 11.11.