alidots / rawfile-localpv

Dynamic Hostpath Local Volumes for Kubernetes Stateful workloads

RawFilePV

Kubernetes LocalPVs on Steroids

Prerequisite

• Kubernetes: 1.21+

Install

helm install -n kube-system rawfile-csi ./deploy/charts/rawfile-csi/

Usage

Create a StorageClass with your desired options:

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: my-sc
provisioner: rawfile.csi.openebs.io
reclaimPolicy: Delete
volumeBindingMode: WaitForFirstConsumer
allowVolumeExpansion: true

Features

• Direct I/O: Near-zero disk performance overhead
• Dynamic provisioning
• Enforced volume size limit
• Access Modes
  • ReadWriteOnce
  • ReadOnlyMany
  • ReadWriteMany
• Volume modes
  • Filesystem mode
  • Block mode
• Volume metrics
• Supports fsTypes: ext4, btrfs, xfs
• Online expansion: If fs supports it (e.g. ext4, btrfs, xfs)
• Online shrinking: If fs supports it (e.g. btrfs)
• Offline expansion/shrinking
• Ephemeral inline volume
• Snapshots: If the fs supports it (e.g. btrfs)

Motivation

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.

License