How do I add "alternate links" to VxVM volume?

[Stuart Abramson_2]

I created a VxVM data group, disk media, volume, etc, with VxVM commands on "PV" c15t4d3.

# vxdisksetup -i c15t4d3
# vxdg init otdg otdg01=c15t4d3
# vxdisk list | grep dg
# vxassist -g otdg make otlv01 1g
# vxprint -v
# find /dev -name otlv01
# newfs - o largefiles /dev/vx/rdsk/otdg/otlv01
# vi /etc/fstab
# add entry
# mkdir /u01
# mount -a

But now I want to add the 2nd path to the disk: c14t4d3.

How do I do that?

[Steven E. Protter]

The answer should be in here.

http://docs.hp.com/cgi-bin/fsearch/framedisplay?top=/hpux/onlinedocs/5187-1372/5187-1372_top.html&con=/hpux/onlinedocs/5187-1372/00/00/22-con.html&toc=/hpux/onlinedocs/5187-1372/00/00/22-toc.html&searchterms=links%7cvxvm%7calternate&queryid=20030916-123919

See dynamic multipathing section.

SEP

[Jeff Schussele · Accepted solution]

Hi Stuart,

Alt Links, as they're known in LVM are termed DMP (Dynamic Multi Pathing) in VxVM. And now comes the gotcha - They're *only* available IF you purchased the full-blown VxVM. Once you do & you get the codeword, you then install the B9116AA product & they show up automatically.
You'd see them in a
vxdisk list c15t4d3
command.
This is one of the downsides of running the base VxVM product.

Rgds,
Jeff

[Stuart Abramson_2]

Thanks, Jeff.

But since I will be installing EMC PowerPath in the next week or so, that should take care of multi-pathing.. Right??

(This is a new server and we are still ordering the PowerPath.)

Stuart

[Jeff Schussele]

You are correct Stuart.

And PowerPath is a much better product as it includes load-balancing as well as alternate links.
Before you order have your EMC Rep verify that the version of PowerPath you'll be ordering fully supports VxVM 3.5 or whatever VxVM ver you're running. And you might want to consult both the Veritas & EMC release notes for any unseen gotchas.

Rgds,
Jeff

[Paul Cross_1]

I was under the impression that DMP does both load balancing and path failover... Can someone verify that?

[Jeff Schussele]

Hi,

Maybe so, but I've done no benching to verify that, let alone it's performance from a balancing perspective.
But from what I've seen, PowerPath is very good at it solely because it's working from a downstream perspective. Always a better view there.

My 2 cents,
Jeff

[Duncan Edmonstone]

This was my understanding of the DMP differences between VxVM and PowerPath/SecurePath/HDLM/AutoPath etc.

- VxVM load balances via simple round robin IO requests except fo certain sequential IO operations:

Sequential I/O starting within a certain range is sent down the same path in order to benefit from disk track caching. Large sequential I/O that does not fall within the range is distributed across the available paths to reduce the overhead on any one path.

-PowerPath/SecurePath/HDLM/AutoPath can also load balance in this round-robin fashion, or they can actually examine the IO queue depths on particular paths to determine the best to use.

HTH

Duncan

[Jakes Louw]

I also understand that PowerPath has hooks into the kernel, so there is a risk that applying new LVM patches could be problematic.