[ovirt-devel] Re: Improving VM behavior in case of IO errors

On Wed, Jul 29, 2020 at 2:30 PM Shubha Kulkarni <shubha.kulkarni(a)oracle.com&gt; wrote: > > Thanks for the feedback Nir. > > I agree in general that having an additional engine config for disk > level error handling default would be the right way. It would be good to > decide the granularity. Would it make sense to have this for a specific > disk type like lun or would you prefer to make it generic for all types? This must be for a specific disk type, since for thin images on block storage we cannot support propagating errors to the guest. This will break thin provisioning.

Handling the LUN use case first seems like the best way, since in this case we don't manage the LUN and we don't support resuming paused using LUNs yet, so propagating the error may be more useful. Managed Block Storage (cinderlib based disks) are very much like direct LUN. In this case we do manage the disks on the server, but otherwise we don't support anything on the host (e.g. monitoring, resuming paused VMs) so propagating the error like direct LUNs may be more useful. Images are a bigger problem since thin disks cannot support propagating errors but preallocated disks can. But once you create a snapshot prealocated disks behave exactly like thin disks because they are the same. Snapshots are also created automatically in for preallocated images, for example during live storage migration, and deleted automatically after the migration. So you cannot assume that having only preallocated disks is good for propagating errors. Even if you limit this option to file based storage, this is going to break when you migrate the disks to block storage. Nir > > Thanks, > > Shubha > > On 7/28/2020 2:03 PM, Nir Soffer wrote: > > On Tue, Jul 28, 2020 at 4:58 AM Shubha Kulkarni > > <shubha.kulkarni(a)oracle.com&gt; wrote: > >> Hello, > >> > >> In OVirt, we have a property propagate_error at the disk level that > >> decides in case of an error, how this error be propagated to the VM. > >> This value is maintained in the database table with the default value > >> set as Off. The default setting(Off) results in a policy that ends up > >> pausing the VM rather than propagating the errors to VM. There is no > >> provision in the UI currently to configure this property for disk > >> (images or luns). So there is no easy way to set this value. Further, > >> even if the value is manually set to "On" in db, it gets overwriiten by > >> UI everytime some other property is updated as described here - > >> https://bugzilla.redhat.com/show_bug.cgi?id=1669367 > >> > >> Setting the value to "Off" is not ideal for multipath devices where a > >> single path failure causes vm to pause. > > Single path failure should be transparent to qemu. multipath will fail over > > the I/O to another path. The I/O will fail only if all paths are down, and > > (with the default configuration), multipath path checkers failed 4 times. > > > >> It puts serious restrictions for > >> the DR situation and unlike VMWare * Hyper-V, oVirt is not able to > >> support the DR functionality - > >> https://bugzilla.redhat.com/show_bug.cgi?id=1314160 > > Alghouth in this bug we see that failover that looks successful from multipath > > and vdsm point of view ended in paused VM: > > https://bugzilla.redhat.com/1860377 > > > > Maybe Ben can explain how this can happen. > > > > I hope that qemu will provide more info on errors in the future. If we had a log > > about the failure I/O it could be helpful. > > > >> While we wait for RFE, the proposal here is to revise the out of the box > >> behavior for LUNs. For LUNs, we should propagate the errors to VM rather > >> than directly stopping those. This will allow us to handle short-term > >> multipath outages and improve availability. This is a simple change in > >> behavior but will have good positive impact. I would like to seek > >> feedback about this to make sure that everyone is ok with the proposal. > > I think it makes sense, but this is just a default, and it cannot work > > for all cases. > > > > This can end in broken VM with read only file system that must be > > rebooted, while > > with error_policy="stop", failover may be transparent to the VM even > > if it was paused > > for a short time. > > > > I would start by making engine defaults configurable using engine > > config, so different > > oVirt distributions can use different defaults. > > > > Nir > > >