Regarding Alternate Link

[Marvike]

As I Am new to hp-unix and we are using EMC SAN storage

I want to know how if one HBA card fails how Second HBA card use the alternate device file.
In case if I have alternate link to LUN assign from EMC SAN Storage as follow.

--- Physical volumes ---
PV Name /dev/dsk/c2t1d3
PV Name /dev/dsk/c4t1d3 Alternate Link
PV Status available
Total PE 17425
Free PE 5905
Autoswitch On
Proactive Polling On

[Torsten.]

Without additional multipathing software LVM will just use the alternate link if the primary fails.

[Steven E. Protter]

Shalom,

Built in alternate link requires two paths to the LUN which you have.

If the primary hba fails, the system and any data and file systems sitting on it should continue to function through the failure.

SEP

[James R. Ferguson]

Hi:

The concept of alternate links (also known as 'pvlinks') is that LVM uses them for high-availablity. While no load-balancing occurs between two links; that is, only one is used at any given time, a failure of one path means that the alternate path handles I/O. This occurs without any intervention on your part.

Regards!

...JRF...

[Torsten.]

And BTW ... because of "Proactive Polling" LVM is using always the "best" path to the disk.

[Berd]

Bhujang,

The first disk is presented down controller c2 and the alternate path is via c4.

If you issue a command like ioscan -fnkCctl you will see entries for all the ctl devices on your machine. Take note of the H/W path for /dev/rscsi/c4t0d0 and c2t0d0.

Then issue a ioscan -fnkCfc to identify your HBA cards, and you will be able to identify the HBA card on the same hardware path.

Regards,
Berd

[Ganesan R]

Hi,

That is why it is called as "Alternate links". It is the feature of lvm that it will switch over to the alternate link whenever primary path fails. Load balancing also possible using alternate links.

But you should have added the alternate links into volume group. Otherwise switch over will not happen eventhough the paths are available.

[TTr]

If one HBA fails, LVM will use the other one without having to do anything.

If you have more than one LUN ensure that in your volume groups half of them have c2 as the primary and c4 as the alternate pathes and the other half of the LUNs have c4 as the primary and c2 as the alternate. This way you get i/o on both HBAs.