[dm-devel] Designing a new prio_callout

[Ethan John]

Hannes, thanks again for your help with this.

I haven't noticed that failback does the right thing, but I'll try it out
again. Could be something we're doing wrong. In any case, there's very
little documentation on all this, and I'm trying to develop some kind of
strategy for our Linux customers to use until we get ALUA implemented.

Being able to set path priorities manually would be ideal, but it seems like
this is impossible, right?

Here's the situation we have right now. I initiate two connections to one
target, across two sessions with two different IPs, with two LUs. Multipath
looks like this:
mpath45 (20002c9020020001a00151b6b46bb57b0) dm-1 company,iSCSI target
[size=15G][features=0][hwhandler=0]
\_ round-robin 0 [prio=1][active]
 \_ 22:0:0:1 sdc 8:32  [active][ready]
\_ round-robin 0 [prio=1][enabled]
 \_ 23:0:0:1 sde 8:64  [active][ready]
mpath44 (20002c9020020001200151b6b46bb57ae) dm-0 company,iSCSI target
[size=15G][features=0][hwhandler=0]
\_ round-robin 0 [prio=1][enabled]
 \_ 22:0:0:0 sdb 8:16  [active][ready]
\_ round-robin 0 [prio=1][enabled]
 \_ 23:0:0:0 sdd 8:48  [active][ready]

Note that there are only two active sessions:
# iscsiadm -m session
tcp: [20] 10.53.152.22:3260,1 iqn.2001-07.com.company:qaiscsi2:blah1
tcp: [21] 10.53.152.23:3260,2 iqn.2001-07.com.company:qaiscsi2:blah1

So the result is that all activity is routed to the first session that was
initiated. I want to change the priorities of the paths to allow for traffic
to go to the first IP for mpath45 and the second IP for mpath46.

Obviously ALUA is the way to go for this in the future, but we won't have
the resources to implement that, so I'm looking for an interim solution that
will scale to thousands of clients. Right now, the only thing I can tell
people is to manually initiate connections to certain targets through
certain IP addresses -- basically, doing the load balancing themselves. Is
there a better way?

On 8/10/07, Hannes Reinecke <hare at suse.de> wrote:
>
> Ethan John wrote:
> > Is it possible to manually set the priority of a path after a connection
> has
> > been established?
> >
> > Since we're doing failover-only (only 1 active path at a time), it would
> be
> > nice to tell users that they can manually reset priority after a
> failure.
> > For example, in a configuration with two paths, where one is active and
> the
> > other is passive for two differeent volumes, a failure of one path will
> > result in all traffic going through the one remaining path. After the
> second
> > path comes back up, all traffic will still be written to the first path
> > (paths are not rebalanced after a failure).
> >
> Not necessarily. There is the keyword 'failback', which can be set to
> IMMEDIATE, causing all paths to fail back to the original path once it
> comes back.
>
> And as you don't actually need to send any commands for facilitate
> the failover I doubt you'd need to develop your own hardware handler.
>
> The existing tweaks should be enough, I think.
>
> Cheers,
>
> Hannes
> --
> Dr. Hannes Reinecke                   zSeries & Storage
> hare at suse.de                          +49 911 74053 688
> SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
> GF: Markus Rex, HRB 16746 (AG Nürnberg)
>
> --
> dm-devel mailing list
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>



-- 
Ethan John
http://www.flickr.com/photos/thaen/
(206) 841.4157
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