On 07/15/2016 05:59 AM, Nitesh Konkar wrote:

Link: http://wiki.libvirt.org/page/NPIV_in_libvirt Topic: Virtual machine configuration change to use vHBA LUN

Sorry for the delay on this... trying to "page in" NPIV details and configure environments is time consuming - time I haven't been able to carve out...

There is a NPIV storage pool defined on two hosts and pool contains a total of 8 volumes, allocated from a storage device.

Source:

# virsh vol-list poolvhba0 Name Path ------------------------------------------------------------------------------ unit:0:0:0 /dev/disk/by-id/wwn-0x6005076802818bda3000000000000366 unit:0:0:1 /dev/disk/by-id/wwn-0x6005076802818bda3000000000000367 unit:0:0:2 /dev/disk/by-id/wwn-0x6005076802818bda3000000000000368 unit:0:0:3 /dev/disk/by-id/wwn-0x6005076802818bda3000000000000369 unit:0:0:4 /dev/disk/by-id/wwn-0x6005076802818bda300000000000036a unit:0:0:5 /dev/disk/by-id/wwn-0x6005076802818bda3000000000000380 unit:0:0:6 /dev/disk/by-id/wwn-0x6005076802818bda3000000000000381 unit:0:0:7 /dev/disk/by-id/wwn-0x6005076802818bda3000000000000382 --------------------------------------------------------------------

Destination: -------------------------------------------------------------------- # virsh vol-list poolvhba0 Name Path ------------------------------------------------------------------------------ unit:0:0:0 /dev/disk/by-id/wwn-0x6005076802818bda3000000000000380 unit:0:0:1 /dev/disk/by-id/wwn-0x6005076802818bda3000000000000381 unit:0:0:2 /dev/disk/by-id/wwn-0x6005076802818bda3000000000000382 unit:0:0:3 /dev/disk/by-id/wwn-0x6005076802818bda3000000000000367 unit:0:0:4 /dev/disk/by-id/wwn-0x6005076802818bda3000000000000368 unit:0:0:5 /dev/disk/by-id/wwn-0x6005076802818bda3000000000000366 unit:0:0:6 /dev/disk/by-id/wwn-0x6005076802818bda300000000000036a unit:0:0:7 /dev/disk/by-id/wwn-0x6005076802818bda3000000000000369 --------------------------------------------------------------------

As you can see in the above output,the same set of eight LUNs from the storage server have been mapped, but the order that the LUNs are probed on each host is different, resulting in different unit names on the two different hosts .

Yep - I see what you're pointing out. As I think I've pointed out before to you either in private chat or email, libvirt relies on systemd for population of the /dev/disk/by-{path|id|uuid}. Furthermore, there have been issues fixed in the "by-path" logic (as of systemd-219), but I believe you asked about a by-id issue for which I forget the details. I did find an external bz for the by-path issue, see: https://bugzilla.redhat.com/show_bug.cgi?id=1032218 (although it's RHEL6 based). There's also a github patch referenced in the private bz as: https://github.com/lnykryn/systemd-rhel/commit/69fc7c636b2b962c386e835ca33c4...

If the the guest XMLs is referencing its storage by "unit" number then is it safe to migrate such guests because the "unit number" is assigned by the driver according to the specific way it probes the storage and hence when you migrate these guests , it results in different unit names on the destination hosts. Thus the migrated guest gets mapped to the wrong LUNs and is given the wrong disks. The problem is that the LUN numbers on the destination host and source host do not agree. Example, LUN 0 on source_host, for example, may be LUN 5 on destination_host. When the guest is given the wrong disk, it suffers a fatal I/O error. (This is manifested as fatal I/O errors since the guest has no idea that its disks just changed out under it.)The migration does not take into account that the unit numbers do match on on the source and destination sides.

So, should libvirt make sure that the guest domains reference NPIV pool volumes by their globally-unique wwn instead of by "unit" numbers?

The guest XML references its storage by "unit" number.

Eg:- <disk type='volume' device='lun'> <driver name='qemu' type='raw' cache='none'/> <source pool='poolvhba0' volume='unit:0:0:0'/> <backingStore/> <target dev='vdb' bus='virtio'/> <alias name='virtio-disk1'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/> </disk>

I am planning to write a patch for it. Any comments on the above observation/approach would be appreciated.

I suppose I'd have to see the patch(es) in order to fully understand what your proposal is. It seems as though what you want to do is allow/add a 'wwn' attribute instead of a 'volume' attribute so that regardless of whether the pool is "by-id" or "by-path", a search could be done on the 'wwn' and a path to the volume created. I think that would work, but seeing the details would certainly help. Interestingly there's a thread on libvirt-users: https://www.redhat.com/archives/libvirt-users/2016-August/msg00063.html which is taking a slightly different use case approach for NPIV/vHBA. In that case, the desire seems to be more multiple paths on a single host and being able to force failover between the paths. I'm not quite sure why that failover just doesn't work behind the scenes, but it also seems to point out a need/desire to have the vHBA be exposed in the guest and let the guest "find" the volumes it wants. John

Thanks,

Nitesh.

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