Thanks for the info Chris. I think that ties it all nicely together for
me!

Kevin

-----Original Message-----
From: rhelv5-list-bounces@(protected)
[mailto:rhelv5-list-bounces@(protected)
Sent: Monday, April 21, 2008 6:06 PM
To: Red Hat Enterprise Linux 5 (Tikanga) discussion mailing-list
Subject: Re: [rhelv5-list] HP hardware + Qlogic HBA + Hitachi USP-V +
failover?

Collins, Kevin [Beeline] wrote:
> Hi,
>
>      I've been on this list (and RHEL3/4 lists) for quite some
time.
> I've noted over that time that there are different ways to do things
in
> regards to using Qlogic HBAs with RHEL on HP hardware. It would appear
I
> have my choice of:
>
> 1) RedHat provided drivers (qla2xxx in my case)
> 2) HP provided drivers, which I must compile (and which I have
gathered
> over time invalidate my support from RedHat)
> 3) Qlogic provided drivers
>
> I appear to have things working with the basic multipath setup
provided
> and the RedHat provided driver. However, this is my first foray in SAN

> with Linux, and my ultimate goal is to provide a highly reliable NAS
> (NFS/CIFS) server via GFS and RedHat Cluster Suite. I want to make
sure
> I can failover to alternate paths if need be.
>
> So, my question is, will the default setup of multibus, round-robin be

> enough to handle a path failure? If so, why would I want to set up a
> "failover" vs a "multibus"?
>
> I have also read that I can use MD (mdadm) to do failover - where does

> that fit in.
>
> Please feel free to share you opinions/experiences as well as links to

> anything you think I might find useful.
>
> I have already read the RedHat doc
>
(_http://www.redhat.com/docs/manuals/enterprise/RHEL-5-manual/en-US/RHEL
510/DM_Multipath_)
> and don't feel much more educated than before I started :)
>
> Thanks,
>
> Kevin

The "failover" configuration *only* does failover, and doesn't give you
any link
aggregation. It generally only makes sense with active/passive arrays.
If
multibus is working for you, keep using it, and enjoy your performance
boost.

mdadm multipathing is obsolete now that we have dm-multipath. It's
still
supported, since there are plenty of existing deployments that use it,
but if
you have the choice, you should use dm-multipath.

As for drivers, you're probably best off using the one in included in
the
distribution. The only interesting thing about the one from Qlogic is
that it
enables support for hardware multipathing, which is disabled in our
build
because it's prone to performance problems under load. The Qlogic
driver is
popular with RHEL 3, since there's no dm-multipath there, and mdadm
doesn't give
you link aggregation, but in RHEL 4 and newer, there's not much benefit
to using
it. I believe the HP version is basically the same thing as the Qlogic
version,
though HP doesn't always have the latest and greatest version from
Qlogic.

As for supportability, we won't slam the door on you if you're using
3rd-party
drivers, but it can greatly increase time to resolution since we're less

familiar with the nuances of our partners' versions than ours, and
coordinating
with them on the back end takes longer than doing it in-house. Back in
the RHEL
3 days, this was worth it to some people to get the benefit of hardware
multipathing, but now that we have dm-multipath, which tends to hold up
better
under high load than the hardware multipathing, there's not much point.

-- Chris

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