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Re: Alternate links on LVM....

 
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Jimmy Rogers
Frequent Advisor

Alternate links on LVM....

Have an L-class server running 11.0 with SC-10 disk trays.

My question is, do alternate links attached to a tray provide load-balancing or only redundancy? I tried searching through some white papers and there seemed to be hints that the answer is dependant on whether the disk tray in question supports active/active status as opposed to active/passive, but am unsure if this applies to LVM, or VxVM only? Or is some other software required to load-balance?
9 REPLIES 9
James R. Ferguson
Acclaimed Contributor

Re: Alternate links on LVM....

Hi Jimmy:

With LVM, alternate links do *not* load-balance. The alternate link is used only in the event the primary one fails. Up to seven alternate links are supported by only one can be promoted to a primary upon failure of the primary.

With VxVM, dynamic load-balancing is supported.

Regards!

...JRF...
melvyn burnard
Honored Contributor

Re: Alternate links on LVM....

Alternate or PV links do not supply load balancing, but redundancy, using normal LVM.

I believe VxVM has the capability to do active/active load balancing, but I am unsure whether this is currently supported on HP-UX, although from this manual:
http://docs.hp.com/cgi-bin/fsearch/framedisplay?top=/hpux/onlinedocs/B7961-90025/B7961-90025_top.html&con=/hpux/onlinedocs/B7961-90025/00/00/13-con.html&toc=/hpux/onlinedocs/B7961-90025/00/00/13-toc.html&searchterms=vxvm&queryid=20020226-104003

It seems there is a section on Dynamic Multi Pathing


HTH
My house is the bank's, my money the wife's, But my opinions belong to me, not HP!
Mark Mitchell
Trusted Contributor
Solution

Re: Alternate links on LVM....

PV links are for redundancy but can be used to load balence. If you do a vgdisplay -v on a vg you will see 2 addresses for the disk. The first is used as a primary and the second is backup. If you vgreduce the first address
"vgreduce /dev/dsk/c10/t1/d0 vg02"
the second address becomes the primary. The next step is to vgextend the same disk back
"vgetend /dev/dsk/c10/t1/d0 vg02"
So now the old primary is the backup pvlink. I use this to force single contoler units to use both controlers by doing this to half of the disks. That way there is still redundance if either controler fails but performance is increased if you know where to split the disks.
Obiously on a striped system this is a no-brainer
Sandip Ghosh
Honored Contributor

Re: Alternate links on LVM....

It can be done by defining the primary link through the alternate link. Suppose it is connected through Primary link and the alternate link.

Then 5 disks has to be configured through the primary link address and the other 5 disks has to be configured through the alternate link addresses as primary address. In that case the first 5 disk will be accessed by the primary link and the second set of 5 disks will be addressed by the alternate link.

Sandip
Good Luck!!!
Jimmy Rogers
Frequent Advisor

Re: Alternate links on LVM....

James...correct answer, which would force me to request funds from management, so a 9.

Melvyn, again, correct and with a link, so an 8 for being second with the info.

Mark, I like your poor-man's load-balancing scheme, so you get a 10 for thinking out-of-the-box!

Thanks for the help all of you!
Sanjay_6
Honored Contributor

Re: Alternate links on LVM....

Hi Jimmy,

LVM alternate links only provide redundancy and do not provide any load balancing,

//Quote //

HP-UX does not do load balancing with LVM across pvlinks. LVM will always communicate through the primary path. LVM will only use the alternate path if the primary path becomes unavailable.

VxVM 3.1 will allow load balancing down both paths in what is called active/active mode on the disk. VxVM is available with HP-UX 11i (11.11) only and cannot be used for the root volume group at this release.

Typically, the primary and alternate links are across two different controllers; controller A and controller B. If a disk array had 6 luns, then typically the administrator would set 3 to have primary paths on controller A alternate paths on controller B. The remaining three would be set up with primary paths on controller B and alternate paths on controller A. During normal operation 3 luns would communicate on one controller card and the other 3 using the second controller. The alternate paths would only be used if the primary paths became unavailable for some reason.

//EndQuote//

You can also try the links below for more opinions,

http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0x62e65f260cafd4118fef0090279cd0f9,00.html

http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0x54d7f841489fd4118fef0090279cd0f9,00.html

Hope this helps.

regds
Mark Mitchell
Trusted Contributor

Re: Alternate links on LVM....

I have never had a problem with this, and it has helped gain performance in older 12h and mod 20 arrays that are limited to one primary and one backup controler. By reducing and extending the outage to move to the altern link is made. And if you stripe disks on one controler and mirror on the second controler it gives great performance. Just because I haven't had time to build up points here doesn't mean that it doesn't work well.
Jakes Louw_1
Frequent Advisor

Re: Alternate links on LVM....

I'm running a billing app with roughly 100 TB on hand. I use the PVLINKS to load-balance by alternating Primary and Alternate paths to the same EMC LUN.
I've monitored this extensively with Glance, and can confirm that the I/O request DEFINITELY follows the new path, as I can see that the I/Os are registered against the "new" device path. In this way, I get to exercise all of the FC cards instead of just half. I cannot provide benchmark results, but batch processing is definitely faster using the PVLINKS.
Sven! Wagner
New Member

Re: Alternate links on LVM....

I do it the same way - the half of the disk on one and the other half on the other controller.
Now I have a cluster environment - when i change ag volumegroup on one server and import this volumegroup on the other server - my alternating is gone.
If i want the alternating again on the other server I must activate this vg and run my "alternatescript" - but I din't want to to activate the VG because i will have the package (vg) up and running on the first node.
Have anyone a resolution for this? Mybe I can change the /etc/lvmtab direct?