/stateful-storage

name: stateful-storage
description: "Creates persistent storage for stateful workloads on Control Plane. Use when the user asks about volumes, volume sets, disks, mounting storage, snapshots, volume expansion, filesystems, shared storage, or backups."

Stateful Storage & VolumeSets

> **Tool availability:** the snapshot tools (`create_volumeset_snapshot`, `list_volumeset_snapshots`, `restore_volumeset_snapshot`, `delete_volumeset_snapshot`), `shrink_volumeset`, and `delete_volumeset_volume` are in the `full` MCP toolset; `create_volumeset`, `update_volumeset`, `mount_volumeset_to_workload`, `expand_volumeset`, and the generic `list_resources`/`get_resource`/`delete_resource` reads are in `core`. If a `full` tool is not advertised, reconnect the MCP server with `?toolsets=full` or use the `cpln` CLI fallback.

A **VolumeSet** is GVC-scoped persistent storage for workloads. The `workload` skill covers the basics (stateful type, reserved mount paths, the 15-volume limit, create-then-verify); this skill is the full volume-set detail. The one trap that drives most rework: **`fileSystemType` and `performanceClass` are immutable** (a PATCH that changes either returns HTTP 400) — to change either you must create a new volumeset, and the old data does not carry over. Choose both at creation.

**Most databases don't need this skill:** `template-catalog` installs Postgres, Redis, MySQL, MongoDB, and more with the volumeset, snapshots, and credentials already wired — hand-build only for a custom app or an unsupported engine.

Filesystem types and performance classes

| Filesystem | Access | Workloads | Volumes provisioned | Snapshots / shrink / delete-volume | |---|---|---|---|---| | `ext4` | read-write-once | one stateful/vm workload | one per replica, per location | yes | | `xfs` | read-write-once | one stateful/vm workload | one per replica, per location | yes | | `shared` | read-write-many | any workload type, many at once | one per location (shared by all replicas) | no — expand only |

| Performance class | Min | Max | Filesystems | |---|---|---|---| | `general-purpose-ssd` | 10 GB | 65536 GB | ext4, xfs | | `high-throughput-ssd` | 200 GB | 65536 GB | ext4, xfs | | `shared` | 10 GB | 65536 GB | shared (auto-set) |

When `fileSystemType: shared`, `performanceClass` is **auto-set to `shared`** — do not specify another. Data is **per-location** and never replicated across locations; for cross-location redundancy, replicate at the application layer (e.g. WAL streaming).

Create a volumeset

Use `mcp__cpln__create_volumeset` (MCP create/mount tools default `fileSystemType` to **xfs** and `performanceClass` to `general-purpose-ssd`; the raw API/`cpln apply` default is **ext4**). YAML for IaC / CLI fallback:

kind: volumeset
name: pg-data
gvc: GVC
spec:
  fileSystemType: ext4
  performanceClass: general-purpose-ssd
  initialCapacity: 20          # GB; within the class min/max and <= autoscaling.maxCapacity
  autoscaling:
    maxCapacity: 100
    minFreePercentage: 20      # 1-100
    scalingFactor: 1.5         # >= 1.1
  snapshots:
    schedule: "0 2 * * *"      # cron; no more than once per hour
    retentionDuration: 7d      # float + d/h/m; tool default 7d

Apply with `cpln apply -f volumeset.yaml --gvc GVC`. Update mutable fields (capacity, autoscaling, snapshot policy, tags) with `mcp__cpln__update_volumeset`.

Autoscaling

**Reactive**: a background job checks volumes about once a minute; when free space falls below `minFreePercentage` it resizes the volume to hold current usage at that margin, scaled up: `new_capacity = ceil(usedGB / (1 - minFreePercentage/100) * scalingFactor)`, capped at `maxCapacity`. Both fields are required, or autoscaling does nothing.

**Predictive** runs the same formula on *projected* usage (from the recent growth rate) to expand ahead of demand; the larger of the reactive and predictive targets wins. Requires `minFreePercentage > 0` and `scalingFactor >= 1.1`:

  autoscaling:
    maxCapacity: 200
    minFreePercentage: 20
    scalingFactor: 1.5
    predictive:
      enabled: true            # default false
      lookbackHours: 24        # 1-168
      projectionHours: 6       # 1-72
      minDataPoints: 10        # 2-100
      minGrowthRateGBPerHour: 0.01
      scalingFactor: 1.2       # >= 1.1; defaults to the parent scalingFactor

Mount to a workload

Mount with `mcp__cpln__mount_volumeset_to_workload` — it attaches to the **first container** and creates the volumeset if missing (create-only defaults, ignored when the volumeset already exists: path `/mnt/{volumesetName}`, filesystem `xfs`, class `general-purpose-ssd`). Volume URI is `cpln://volumeset/VOLUMESET`.

• **ext4/xfs require a `stateful` or `vm` workload** (mounting on serverless/standard returns HTTP 400); `shared` mounts on any type. Workload type is immutable — see "Migrating to stateful" below.
• Up to **15 volumes** per container. **Reserved mount paths** (rejected): `/dev`, `/dev/log`, `/tmp`, `/var`, `/var/log`.
• `recoveryPolicy`: `retain` (default — reuse an existing volume's data on a new replica) or `recycle` (start fresh).
• `path` is required for non-vm workloads and rejected for `vm` (VM disks use `name`/`bus`/`bootOrder` instead).
• Stateful workloads give each replica a stable index and its own volume; `spec.loadBalancer.replicaDirect` (stateful-only) exposes per-replica endpoints — see the `workload` skill.

kind: workload
name: pg
gvc: GVC
spec:
  type: stateful
  containers:
    - name: postgres
      image: //image/postgres:16
      ports:
        - number: 5432
          protocol: tcp        # http | http2 | grpc | tcp — a DB is tcp, not http
      volumes:
        - uri: cpln://volumeset/pg-data
          path: /var/lib/postgresql/data

Snapshots

Snapshots are **ext4/xfs only — never `shared`**. Automatic policy lives in `spec.snapshots`: `createFinalSnapshot` (default `true` — a snapshot is taken before any volume in the s