Re: [linux-lvm] SAN Storage/mirror/lvm

[Thomas Meller <thomas.meller@gmx.net>]

Cott Lang wrote:

> On the host, I see 4 SCSI devices, two valid, two invalid - I'm
> guessing this is caused because only one storage processor exports
> a particular LUN, so half the paths are invalid until a storage
> processor fails over.
> 
> sdd - SP A Path 1
> sde - SP B Path 1
> sdf - SP A Path 2
> sdg - SP B Path 2
> 
> I have a LUN currently on SP B, so I can use md multipath configured
> with /dev/sde and /dev/sdg, and that seems to work.
> 
> However, two things:
> 
> 1) How do I set this up to handle a failover to SP A?
> 2) Multipath only seems to work in failover mode, is there a way to
> load balance across the two paths?
> 
> Anyone have any hints?  :)

Hello Cott,

your questions and the phenomena you see are likely to be specific to your hardware. I tried
Emulex and QLogic HBA and found very different behaviour.

Multipathing is handled by the storage systems differently from dumb disks. Load balancing
over several physical paths is therefore a complicated task. Most vendors do not really
support it. Some systems suffer from a Host switching paths. Unless you do not use a
supported (read: expensive) Software you should keep your hands off.

Even failover has a 'mechanical' quality: you can hear the door's angles squiek when it
happens.
I found my clusters diving after teardown of a storage's HBA.

Failover while switching the physical path, i.e. switching the HBA takes at least 45 second
on a (tuned) qlogic. Meanwhile the whole computer freezes. The driver seems to switch off
interrupts or something.

Have fun finding your solution.

Thomas