[dm-devel] SAN failover question

[Christophe Varoqui]

On dim., 2011-05-01 at 23:01 +0200, urgrue wrote:
> I've tried all around to find a good solution for my conundrum, without 
> much luck.
> 
> The point is, multipath works nice. Until a bigger disaster comes along, 
> e.g. san or datacenter failure. Of course like most big environments you 
> have a synchronous replica of your SAN. But you have to "do stuff" to 
> get Linux to take that new LUN and get back to work. A reboot, or san 
> rescans, forcibly removing disks and so forth. It's not very pretty.
> 
> So my question is, is there any way to get multipath to treat both the 
> active lun and it's passive replica (usually in readonly or offline 
> state) as one and the same disk? The goal being, if your SAN fails, you 
> merely have to activate your DR replica, and multipath would pick it up 
> and all's well (except for the 30 sec to a few mins of I/O hanging until 
> the DR was online). In essence, you'd have four paths to a LUN - 2 to 
> the active one, 2 to the passive one, which is a different LUN 
> technically speaking (different serials, WWNs, etc), but an identical 
> synchronous replica (identical data, identical state, identical PVID, etc).
> 
You would have to :

1/ setup a customized getuid program instead of the default scsi_id.
(may be based on the pairing id if there is such thing in your context)

2/ set the 'group_by_prio' path grouping policy

3/ develop a prioritizer shared object to assign path priorities based
on the master/slave role of the logical unit. Paths to master get prio
2, paths to slave get prio 1.

May be someone else can comment on the specific ro->rw promotion issue
upon path_group switching. I can't tell if it needs a hw_handler these
days.

-- 
Christophe Varoqui
OpenSVC - Tools to scale
http://www.opensvc.com/