The ability to offload snapshots natively to a NFS filer has been around for a while. Commonly this was used with View Composer Array Integration (VCAI) to rapidly clone VDI images, and occasionally for VMware Cloud Director environments (Fast Clone for vApps). There were some caveats to consider:
- Up until vSphere 7 Update 2 the first snapshot had to be a traditional redo log snapshot.
- VMware blocks storage vMotion for VMs with native snapshots. (You will need to use array replication, and a bit of scripting to move these) which leads to the most important caviot.
- A snapshot.alwaysAllowNative = “TRUE” setting for virtual machines was introduced. This allows the virtual machine in NFS datastore with VAAI plugin to be able to create Native snapshots ignoring its base disk is flat one or not.
- If the Filer refuses to create a snapshot (Most commonly seen when a filer refuses to allow snapshots while doing a background automated clone or replication on some storage platforms), it will revert to redo log. It is worth noting that “alwaysAllowNative” doesn’t actually prevent this fail back behavior.
- Some filers vendors will automatically inject snapshot.alwaysAllowNative = “TRUE” into VMs automatically.
The challenge with this in particular is that it can cause a problem. A Chain that goes from Native, to Redo log back to Native (or Redo Log to Native to Redo log) is invalid and leads to disk corruption!
So what are my options if this is a risk in my environment?
I’ll first off point out that vVols allows offloading of snapshots WHILE retaining support for storage vMotion. It’s fundamentally a bit simpler/more elegant solution to this problem of having natively offloading snapshots.
For most NFS VAAI users this should not be an issue as the filer should just create native snapshots when asked. For platforms that have issues taking native snapshots when other background processes are running, consider disabling that background replication/cloning that is automatically tied to the snapshot tree. If this is not an option consider not using snapshot.alwaysAllowNative and performing full clones, or not using the NFS VAAI clone offload instead. Hopefully in the future there will be a further patch to prevent this issue.