One might have a couple of reasons to consider using node-based (rather than network-based) storage solutions:
- Performance: Almost no network-based storage solution can keep up with baremetal disk performance in terms of IOPS/latency/throughput combined. And you’d like to get the best out of the SSD you’ve got!
- On-premise Environment: You might not be able to afford the cost of upgrading all your networking infrastructure, to get the best out of your network-based storage solution.
- Complexity: Network-based solutions are distributed systems. And distributed systems are not easy! You might want to have a system that is easier to understand and to reason about. Also, with less complexity, you can fix unpredicted issues more easily.
Using node-based storage has come a long way since k8s was born. Right now, OpenEBS’s hostPath makes it pretty easy to automatically provision hostPath PVs and use them in your workloads. There are known limitations though:
- You can’t monitor volume usage: There are hacky workarounds to run “du” regularly, but that could prove to be a performance killer, since it could put a lot of burden on your CPU and cause your filesystem cache to fill up. Not really good for a production workload.
- You can’t enforce hard limits on your volume’s size: Again, you can hack your way around it, with the same caveats.
- You are stuck with whatever filesystem your kubelet node is offering
- You can’t customize your filesystem:
All these issues stem from the same root cause: hostPath/LocalPVs are simple bind-mounts from the host filesystem into the pod.
The idea here is to use a single file as the block device, using Linux’s loop, and create a volume based on it. That way:
- You can monitor volume usage by running df in `O(1)` since devices are mounted separately.
- The size limit is enforced by the operating system, based on the backing file size.
- Since volumes are backed by different files, each file could be formatted using different filesystems, and/or customized with different filesystem options.
