[libvirt-users] Libvirt with multipath devices and/or FC on NPIV

[C.D.]

Hello,

I am trying to find out a best practice for a specific scenario.

First of all I would like to know what is the proper way to set up
multipath, who should care about it the host or the guest. Right now I have
a setup where I have one multipath which sets my host to boot from FC SAN. I
have another multipathed LUN in the host which is essentially a dm which I
attached to a guest, however through virtio. I added the devices through
virsh with pool type of "mpath" and path through /dev/mapper/. So here is my
first question: with such a setup the multipath and eventual fail-over is
taken care by the host OS, right, the guest will not notice is I suppose?
But what about migration. What if I decide to migrate the guest to another
host. How would that work out? WIth shared directory it is easy, you just
have the images in it, but what about such a setup. I can always add the
aforementioned LUN, where the guest resides to another Storage Group on the
SAN where the new host has access and I can make sure that the mpath device
name is persistent across all hosts, but is that the right approach.

Here is another question that is bugging me. I have FC HBA-s on all hosts
and I would like to make a HBA visible through my guest. I stumbled upon a
bug that is documented in redhat's bugzilla that attempted creation of NPIV
on pci_0000_blah_blah instead on the scsi_hostX device, but I fixed that
easily in the xml and although virsh doesn't seem to think I completed the
nodedev-create NPIV_for_my_FC.xml the device is seen as a child in
nodedev-list and what is more important the WWN are seen on the SAN switch.
So this seems to be working OK, however I don't know how to attach this new
shiny device that I created to a guest. Could someone give me a hint? What
is the proper way to attach this new devices to a guest OS and do that
persistently. Do I do that with virsh's attach-device and if so what is the
proper XML format? Should I dump the XML for the newly created NPIV nodedev
and try to attach that? And again with multipathing. What if I decided to
create NPIV on my two FC cards in every host, then do the zoning and attach
those newly created NPIV nodes to the guests? Will that produce the same
effect that such a multipath does on the host? It doesn't really matter if I
would use it for root drive or a shared gfs2 storage between guests, I just
want to know is this possible and is it the right thing to do, or should I
stick to setup in my previous paragraph? And last question: what about
migration with such NPIV FC devices. I can move a guest around, but will
that move my NPIV FC with it's WWN so that I can continue using my zoning (I
think I just realized that this might be impossible because I'm migrating
the guest, not the entire setup of libvirt, but probably I'm wrong and I'm
not aware of a proper method to do it correctly?).

Probably those questions are easy to answer, but please bare with me, I'm
playing with libvirt in an enterprise setup only for the last couple of days
and I would really like to make it right and kick some VMware ass.

By the way, all my hosts are Fedora 14, with kernel 2.6.37 (rebuild of the
one for Fedora 15 from koji, because there was a small glitch with Qlogic's
driver in the stock 2.6.35). Most of my guests will be SL6,CentOS5/6(when it
arrives) and a couple of windows XP guests (but I'm not really concerned
with that).

Thanks for the support in advance

P.S. please keep me in CC, as I'm not on the list. Thank you
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