On Tue, Jul 24, 2018 at 8:30 PM Ryan Bullock <rrb3942@gmail.com> wrote:

... 

Vdsm does monitor multipath events for all LUNs, but they are used only

for reporting purposes, see:

https://ovirt.org/develop/release-management/features/storage/multipath-events/

We could use the events for resuming vms using the multipath devices that

became available. This functionality will be even more important in the next version

since we plan to move to LUN per disk model.

 

 

I will look at doing this. At the very least I feel that differences/limitations between storage back-ends/methods should be documented. Just so users don't run into any surprises.

You can file a bug for documenting this issue.

...

My other question is, how can I keep my VMs with Direct LUNs from pausing during short outages? Can I put configurations in my multipath.conf for just the wwids of my Direct LUNs to increase the ‘no_path_retry’ to prevent the VMs from pausing in the first place? I know in general you don’t want to increase the ‘no_path_retry’ because it can cause timeout issues with VDSM and SPM operations (LVM changes, etc). But in the case of a Direct LUN would it cause any problems?

You can add a drop-in multipath configuration that will change

no_path_retry for specific device, or multiapth.

Increasing no_path_retry will cause larger delays when vdsm try to 

access the LUNs via lvm commands, but the delay should be only on

the first access when a LUN is not available.

 

Would that increased delay cause any sort of issues for Ovirt (e.g. thinking a node is offline/unresponsive) if set globally in multipath.conf? Since a Direct LUN doesn't use LVM, would this even be a consideration if the increased delay was limited to the Direct LUN only?

Vdsm scans all LUNs to discover oVirt volumes, so it will be effected by multipath

configuration applied only for direct LUNs.

Increasing no_path_retry for any LUN will increase the chance to delay some

vdsm flows accessing LUNs (e.g. updating lvm cache, scsi rescan, listing devices).

But the delay happens once when the multipath device loose all paths. The benefit

is smaller chance that a VM will pause or restart because of short outage.

Nir