Alternate links

[Marco Santerre]

I have one real quick question that I unfortunately haven't found the answer anywhere.

I've got four FC attached to an EMC. Each of those FC have the same LUSE mapped out. I could basically have one primary and three alternates. Because this is one of my Prod servers, I can't do it until I'm certain.

Is there a maximum on the number of PV links you can have for one disk ?

thanks in advance

[James Odak · Accepted solution]

from the man file of vgextend

it supports 8 paths .. 1 prime and 7 alt

When a pv_path refers to one of the physical volumes already in the
volume group by a different pv_path name to indicate the use of a
different controller, this new path becomes an alternate link to the
physical volume. When two paths that reference the same disk are
provided in the pv_path list, the order of the paths is important.
The first path becomes the "primary link" to the physical volume, the
second becomes an "alternate link" to the physical volume. The
primary link is the path used to access the physical volume unless the
primary link becomes unavailable in which case LVM automatically
switches to the alternate link to access the physical volume.
Currently LVM supports a maximum of 8 paths to a physical volume (7
alternates and one primary).

[Ian Dennison_1]

From 'man vgextend'.

Currently LVM supports a maximum of 8 paths to a physical volume (7
alternates and one primary).

HPUX 11.11.

I run 4 paths through my XP512 , no problems at all.

Share and Enjoy! Ian

[Marco Santerre]

Of all the places to look for I forgot the man page. Thanks guys!!

[James R. Ferguson]

Hi:

The maximum number of alternate links is eight (8). That is one primary and seven alternate links to one physical volume. Remember that no load-balancing occurs with LVM pvlinks.

Regards!

...JRF...

[Dave Wherry]

There's a misconception that comes up often in the forums when talking about PV links. You didn't mention it but, I'll bring it up anyway.
Remember, they do not do load balancing. They are only for fail-over. When you set them up make sure you do not have all of your primary links on the same HBA. Spread them around. I always suggest striping your logical volumes across the HBAs so you can balance the load. Or, EMC will be happy to sell you Power Path which will do the load balancing.

[Bill McNAMARA_1]

be very careful in the future when you issue vgscans as your PVlinks configuration will be lost.

Also conside testing them, as switching onto an alternate may bottleneck busses/loops try to sensibly determine which should be first alternate/second alternate etc.. You are not obliged to add all links, they are used primarily to remove spofs (which usually will not occur in multiples - fingers crossed)

Later,
Bill

[Steven E. Protter]

Anyone have a cookbook on how to set up PVLINKS?

I need one.

Yes, I looked.

P

[Marco Santerre]

Thank you