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+# Local Volume
+
+## Summary
+Longhorn can support local volume to provide better IO latencies and IOPS.
+
+### Related Issues
+https://github.com/longhorn/longhorn/issues/3957
+
+## Motivation
+### Goals
+- Longhorn can support local volume (data locality=strict-local) for providing better IO latencies and IOPS.
+- A local volume can only have one replica.
+- A local volume supports the operations such as snapshot, backup and etc.
+
+### Non-goals
+- A local volume's data locality cannot be converted to other modes when volume is not detached.
+- A local volume does not support multiple replicas in the first version. The local replication could be an improvement in the future.
+
+## Proposal
+1. Introduce a new type of volume type, a local volume with `strict-local` data locality.
+   - Different than a volume with `best-effort` data locality, the engine and replica of a local volume have to be located on the same node.
+2. Unix-domain socket are used instead of TCP between the replica process' data server and the engine.
+3. A local volume supports the existing functionalities such as snapshotting, backup, restore, etc.
+
+### User Stories
+Longhorn is a highly available replica-based storage system. As the data path is designed for the replication, a volume with a single replica still suffers from high IO latency. In some cases, the distributed data workloads such as databases already have their own data replication, sharding, etc, so we should provide a volume type for these use cases while supporting existing volume functionalities like snapshotting, backup/restore, etc.
+
+### User Experience In Detail
+- The functionalities and behaviors of the volumes with `disabled` and `best-effort` data localities will not be changed.
+- A volume with `strict-local` data locality
+   - Only has one replica
+   - The engine and replica have to be located on the same node
+   - Cannot convert to `disabled` or `best-effort` data locality when the volume is not detached
+   - Can convert to `disabled` or `best-effort` data locality when the volume is detached
+   - Existing functionalities such as snapshotting, backup, restore, etc. are supported
+
+### CLI Changes
+- Add `--volume-name` in engine-binary `replica` command
+   - The unix-domain-socket file will be `/var/lib/longhorn/unix-domain-socket/${volume name}.sock`
+- Add `--data-server-protocol` in engine-binary `replica` command
+   - Available options are `tcp` (default) and `unix` 
+- Add `--data-server-protocol` in engine-binary `controller` command
+   - Available options are `tcp` (default) and `unix` 
+
+## Design
+### Implementation Overview
+#### CRDs
+1. Add a new data locality `strict-local` in `volume.Spec.DataLocality`
+
+#### Volume Creation and Attachment
+- When creating and attaching a volume with `strict-local` data locality, the replica is scheduled on the node where the engine is located.
+- Afterward, the replica process is created with the options `--volume-name ${volume name}` and `--data-server-protocol unix`.
+- The data server in the replica process is created and listens on a unix-domain-socket file (`/var/lib/longhorn/unix-domain-socket/${volume name}.sock`).
+- Then, the engine process of the volume is created with the option `--data-server-protocol unix`.
+- The client in the engine process connects to the data server in the replica process via the unix-domain-socket file.
+
+### Validating Webhook
+- If a volume with `strict-local` data locality, the `numberOfReplicas` should be 1.
+- If a local volume is attached, the conversion between `strict-local` and other data localities is not allowable.
+- If a local volume is attached, the update of the replica count is not allowable.
+
+### Test Plan
+#### Integration tests
+1. Successfully create a local volume with `numberOfReplicas=1` and `dataLocality=strict-local`.
+2. Check the validating webhook can reject the following cases when the volume is created or attached
+   - Create a local volume with `dataLocality=strict-local` but `numberOfReplicas>1`
+   - Update a attached local volume's `numberOfReplicas` to a value greater than one
+   - Update a attached local volume's `dataLocality` to `disabled` or `best-effort`