how many PVLINKS in a VG?

[jithu_1]

Hello folks,

env: HP-UX 11i, the server has
a bunch of disks and each disk has 4 paths to the host(for PVLINKS and manual load balancing across HBA's)

1. Is it non-functional/pointless to have more than just one PVLINK in a VG?? I mean the rest of the 2 in this case are useless in times of failure??
2. How does HP-UX handle, if the primary failed, then the 1st PVLINK it tried also
failed, would it then go and try on the 2nd and 3rd or just stop and error out
after the 1st PVLINK?

thanks in advance
jitu

[Corthouts Carlo]

If the primary failed LVM would switch to the first alternate.
With the default -S autoswitchback setting, LVM would keep on
polling the primary until it returned and switch back. With -S n it would stay on the first alternate. Should the primary and first
alternate both fail then the second alternate would be used. Because the decision on link ordering is based upon the order of the PV names in the lvmtab file, then on reboot or full re-activation the ordering will be reinstated.

[Krishna Prasad]

It doesn't hurt anything and is really just extra safty nets.

Did you just start adminstrating this machine?
If this is a new machine for you it may be possible you are running an application from EMC called powerpath. Are you connected to an EMC frame?

If you are running powerpath you protected against hardware failures but you also get performance bust. Powerpath wiill pick an alternate path if on bus is busy it actually balances I/O accross channels.

[James R. Ferguson]

Hi:

Have a look at the man pages (at 11.x) for 'pvchange', in particular, the 'autoswitch' option.

Regards!

...JRF...

[Roger Baptiste]

hi,

<<1. Is it non-functional/pointless to have more than just one PVLINK in a VG?? I mean the rest of the 2 in this case are useless in times of failure??>>

I use all the available PV links in the VG configuration. It doesn't hurt to use all that you got. Pointless?? I wouldnt think so. I have seen odd instances where the third PVlink was used when the first two failed/switched over. It also depends on the number of Fibre
cards going out of your system; If you are using EMC, each link could be going through different switches. In that case, you would definitely be better off using every link.
So to make it simple, always configure all the pv links.

<<2. How does HP-UX handle, if the primary failed, then the 1st PVLINK it tried also
failed, would it then go and try on the 2nd and 3rd or just stop and error out
after the 1st PVLINK?>>

Yes, hp-ux will try all the pv links.

And as somebody mentioned, if you use EMC disks, they have a product called powerpath which will do the pv link switching dynamically to loadbalance. You wouldnt need to configure the alternates in this case.

ANother point, to get better load balancing,
use the schema in a round robin fashion.

HTH
raj