[dm-devel] Designing a new prio_callout

[Hannes Reinecke]

Ethan John wrote:
> For the record, setting rr_min_io to something extremely large (we're using
> 2 billion now, since I'm assuming it's a C integer) solves the immediate
> problem that we're having (overhead in path switching causing poor
> performance). Telling people to use mpath_prio_random is still less than
> ideal for any small number of iSCSI targets, but it a better short-term
> solution for us than nothing.
> 
In setting rr_min_io to something extremely large you effectively
disable the round-robin scheduler in multipathing.
That's okay for the failover scenario you have (as you only have
one path per group), but whenever you have more than one path
in a group that wouldn't work anymore.

> On 8/10/07, Ethan John <ethan.john at gmail.com> wrote:
>> 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.
>>
That's a matter of the IP routing. Having both target on the same (sub-) net
doesn't work very well with multipathing. Please setup your system with
each iSCSI Target port in a different subnet eg

10.53.152.22:3260,1 iqn.2001-07.com.company:qaiscsi2:blah1
10.53.153.22:3260,2 iqn.2001-07.com.company:qaiscsi2:blah1

then you'll have one iSCSI target port per subnet and you can actually
do failover etc.

>> 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?
>>
No, not really. But I'm not a network guru. You may want to ask on
the open-iscsi mailing list.

And you can get all information you need via sysfs, so it should
be possible to create a script like Stefan Bader suggested.

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)