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Old 05-01-2011, 09:01 PM
urgrue
 
Default SAN failover question

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).



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Old 05-01-2011, 09:40 PM
Christophe Varoqui
 
Default SAN failover question

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
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