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Re: more alternate links, more resilient?

 
zhaogui
Super Advisor

more alternate links, more resilient?

Is it true that the more alternate links, more resilient the system is?

7 REPLIES 7
Vincent Farrugia
Honored Contributor

Re: more alternate links, more resilient?

Hello,

Yes it's true.

This is because if the main link is dead for some reason (could be hardware problem), the link goes to one alternate path. The more alternate paths you have, the more time it takes before all of them fail, making it more resilient.

HTH,
Vince
Tape Drives RULE!!!
zhaogui
Super Advisor

Re: more alternate links, more resilient?

But in any case, the more alternate paths you have,
it might take longer to failover in your server as there will be a timeout
period for each path. So more does not necessary equate better.
Stefan Farrelly
Honored Contributor

Re: more alternate links, more resilient?

Technically, yes. But more importantly the more alternate links the more you should be balancing your VG's across them so increase your max I/O throughput by a huge amount so not only are they for resiliency but increased performance.

eg.
instead of VGxx having;
PV NAME /dev/dsk/c1txdx
/dev/dsk/c2txdx alternate link
PV NAME /dev/dsk/c1txdx
/dev/dsk/c2txdx alternate link
you should balance them as;
PV NAME /dev/dsk/c1txdx
/dev/dsk/c2txdx alternate link
PV NAME /dev/dsk/c2txdx
/dev/dsk/c1txdx alternate link
This way I/O can use c1 and c2 and still have resiliency (backup on the other path) - instead of using c1 only for live and c2 as a backup. In the 2nd example above the potential maximum I/O throughput is doubled!!


Im from Palmerston North, New Zealand, but somehow ended up in London...
Vincent Farrugia
Honored Contributor

Re: more alternate links, more resilient?

Hello,

You asked for better resiliency. It does offer better resiliency. It's true, there are more timeouts, but it's more highly-availiable if there are more alternate links. That's what you asked.

HTH,
Vince
Tape Drives RULE!!!
zhaogui
Super Advisor

Re: more alternate links, more resilient?

But in order to achieve full load-balancing, you need to do LVM striping on c1txdx and c2txdx in case c1 is not fully used. This is true even though you already have disk striping at hardware level in AutoRAID such as VA7400, i.e.
Vincent Farrugia
Honored Contributor

Re: more alternate links, more resilient?

Hello,

We are talking about PATHS here. To the same device. So you CANNOT stripe across the same device.

Here we are talking of THE SAME DEVICE being reached from 2 (or more) different paths.

HTH,
Vince
Tape Drives RULE!!!
James R. Ferguson
Acclaimed Contributor

Re: more alternate links, more resilient?

Hi:

One more point should be made. Stefan's example of a volume group made up of multiple physical volumes is a good one. As he indicated, the I/O performance is improved by choosing different controllers to service different physical volumes. This does *not* mean, however, that both the primary and alternate (secondary) link to a particular physical disk will operate concurrently. LVM does *not* load-balance. The alternate link exists only for high-availability (failover). By contrast, the newer Veritas Volume Manager (VxVM) *does* offer load-balancing.

Regards!

...JRF...