From a quick look into the Changelogs, 2.7 has nothing backwards incompatible that should worry us, 2.6 does however. Specifically
The aio=native option to "-drive" now requires the cache=none option, instead of silently disabling itself for other cache modes. The newly invalid combination had been warning since QEMU 2.3.
We 've set this in the past both as a workaround for QEMU deadlocking issues as well as for performance reasons. Unfortunately it seems like 2.12 (our ganeti version) does not pass cache=none, neither do future versions.
- Patch ganeti to adhere to QEMU 2.6 (this should happen anyway probably and upstreamed but maybe we don't want to wait)
- Set disk_cache=none in all of our clusters. That would be a change from the default of cache=writeback but we are implicitly there anyway as the QEMU 2.6 changelog says.
- Unset disk_aio=native and use the default of threads. That is an option for sure, but might incur some performance penalties
Of the the above, the disk_cache=none seems like the easiest path forward
With cache=none being set in all cluster for unrelated reasons, this is now unblocked. In the meantime jessie-backports has upgrade to 2.8. Fortunately the changelog does not have any worrying items in it. The upgrade will require a round of VM reboots, but otherwise looks ok. I 'll empty an eqiad ganeti host, upgrade to 2.8 and move a few VMs to it for testing.