How to set up alternate disk (alternate link) to a PV in VG

[senthil_kumar_1]

Hi All,

I found that in one of my HPUX server, one VG is having a PV with alternate links.

For example:

--- Physical volumes ---
PV Name /dev/dsk/c6t1d1
PV Name /dev/dsk/c5t0d1 Alternate Link
PV Status available
Total PE 17005
Free PE 0



My Questions:

1)What is the purpose of this?

2)How it is configured?

[Torsten. · Accepted solution]

This is a LUN of an array.

To an array you usually have more than one path.



You (simply) identify the second path and use "vgextend" to add it as an alternate link.

[Benoy Daniel]

Its the same LUN but represented through different path. In case of primary Link failure, the Lun is still accessible through alternate path.

[Torsten.]

This also depends on the OS version and multipathing software in use.

With 11.31 or additional software in previous releases the load is balanced between the other pathes.

[Ismail Azad]

Hi,

This depends on what version you are using as said before. Traditionally this was the concept of something called as PV links which meant that there was dual cabling but load is always transmitted only through one path. The concept of PV links and switchover has been outdated and is deprecated with the design of the new mass storage stack as multipathing is native to 11.31 which distributes I/O load accross all paths by default in a round robin fashion. And I think the other forumers have already answered the question how is it configured?

Regards
Ismail Azad

[Alzhy]

Senthil,

Do a:

diskinfo /dev/rdsk/c6t1d1 (note the r)

it will give you the type of multipathed Disk (which is usually an external HW based Array disk -- SAN these days)

If it is an EVA disk, you can use evainfo utility to find out the other "device paths" of the same disk which can serve as either primary PV or alternate PV. You need to have as many alternate PVs (or PVLINKs) as you have paths to the same disk.

If you are using XP disk, then xpinfo will give you the information to find out what the other "device paths" are.

On HP-UX 11.31, there is what is called Agile or persistent device naming where:

/dev/dsk/diskNN *already* is your PVlinked, multipathed/ load balanced disk and where there is no more need to designate "alternate paths"