EVA 4100 Path B Failover Only

[carboni]

I saw an EVA 4100 where every volume has this Preferred path/mode configuration:
- 1st LUN of a volume: "Path B - Failover Only";
- 2nd or 3rd LUN of a volume: "No preference"
The volumes are presented to two HP-UX servers in cluster (Service Guard). The servers use LVM and Alternate Link. (Every volume is also mirrored on a second EVA).
- I don't understand because also the first LUN isn't configured as "No preference".
- Does "Path B - failover only" mean that active controller is B, and the controller A works only if controller B fails?
Could someone help me to better understand?

[Uwe Zessin]

It just means that when the EVA boots, the first virtual disk is assigned to controller B for management (cache and I/O to disk).

If B fails, A takes over the management. If B comes back, A keeps the management.

If the majority of host I/O goes through A (this depends on the PVlinks path settings), after about a hour the EVA ignores "Path B - Failover Only" and moves management to A.

[Steven Clementi · Accepted solution]

It is simply a setting telling the EVA which controller should "manage" the disk. The disk is accessible across all 4 paths as normal (both controllers are always active).

Typically, you would make these settings to load balance your virtual disks across both controllers if your hosts were not abl to dynamically load balance them.


The setting translates to:

"Make the manageing controller Controller B, and when/if a failure happens... failover to Controler A... but if Controller B comes back online, don't worry about failback."


Steven

[Gordon Sjodin]

I would be more concerned with these settings on an OS that does not support some form of multi path software. You could use this to load ballance your LUN access through the different controllers. VMWare did not support any form of multipath up until recently and these settings would be required to alternate LUN's through the different controllers. I would set them all to "No Preference" since you do have multipath software on the servers.

[Uwe Zessin]

> VMWare did not support any form of multipath up until recently

VMware ESX? I think you confuse 'multipath=be able to deal with multiple data paths to the same disk and do not create any "ghost devices"' with 'load balancing I/Os to the same disk across multiple paths'. 'multipathing' has worked for _years_.

[Gordon Sjodin]

Thanks Uwe,

I am trying to figure out the distinction.

Would it be correct to say "VMWARE would only support traffic down 1 fixed path"?

I think I have multipath misinterpreted. I am assuming "MPIO" implies multipath -- which is an incorrect assumption.

Multipath simply impies an operating systems capability to distinguish mutiple paths to the same disk. -- or NOT to see the same disk as multiple different disks?

Correct?