> ## Documentation Index > Fetch the complete documentation index at: https://docs.basaltic.sh/llms.txt > Use this file to discover all available pages before exploring further. # Attach a data volume to an instance > Attach an existing, available storage volume to the instance and republish the desired spec so the on-host compute-agent hot-plugs the RBD disk into the running domain. The volume must be in the same account and in status `available`. ## OpenAPI ````yaml /api-reference/specs/compute.yaml post /v1/instances/{instance_id}/volumes openapi: 3.0.3 info: title: Basaltic Compute API version: 1.0.0 description: | Virtual machine instances, and the images, flavors, SSH keypairs and instance pools they are built from. Covers the whole instance lifecycle: start, stop, reboot, resize and reinstall. contact: name: Basaltic Support email: ping@basaltic.sh license: name: Proprietary url: https://basaltic.sh/terms servers: - url: https://compute.{region}.basaltic.sh description: Regional API endpoint variables: region: default: sa-saopaulo-1 description: Region code security: - SignatureAuth: [] paths: /v1/instances/{instance_id}/volumes: post: tags: - Compute summary: Attach a data volume to an instance description: | Attach an existing, available storage volume to the instance and republish the desired spec so the on-host compute-agent hot-plugs the RBD disk into the running domain. The volume must be in the same account and in status `available`. operationId: attachInstanceVolume parameters: - $ref: '#/components/parameters/IdempotencyKey' - $ref: '#/components/parameters/InstanceId' requestBody: required: true content: application/json: schema: type: object required: - volume_id properties: volume_id: type: string format: uuid example: 7c9e6679-7425-40de-944b-e07fc1f90ae7 device: type: string description: >- Optional device-name override; auto-picks the next free slot (vdb/vdc/…) when omitted. example: vdb mount_path: type: string description: >- When set, the in-guest agent formats the disk (only if blank) and mounts it at this path. Empty attaches the block device only. example: /data fstype: type: string description: Filesystem for mount_path formatting (default ext4). example: ext4 responses: '202': description: Attachment accepted content: application/json: schema: type: object properties: attachment: type: object properties: instance_id: type: string format: uuid example: 019f24a8-4bbe-7441-93af-6bbac0413943 volume_id: type: string format: uuid example: 7c9e6679-7425-40de-944b-e07fc1f90ae7 device: type: string example: vdb '400': $ref: '#/components/responses/BadRequest' '401': $ref: '#/components/responses/Unauthorized' '403': $ref: '#/components/responses/Forbidden' '404': $ref: '#/components/responses/NotFound' '409': description: Instance or volume not in a state that allows attach content: application/json: schema: $ref: '#/components/schemas/Error' '422': $ref: '#/components/responses/UnprocessableEntity' security: - SignatureAuth: [] components: parameters: IdempotencyKey: name: Idempotency-Key in: header description: >- Optional client-generated key that makes a create replay-safe. Retrying a request with the same key returns the original outcome verbatim instead of creating a duplicate resource. Reusing a key with a different request body is rejected (422); a request whose key is still being processed returns 409. Records are honored for 24 hours. Use a UUID or similarly unique token. required: false schema: type: string maxLength: 255 example: 550e8400-e29b-41d4-a716-446655440000 InstanceId: name: instance_id in: path description: Instance ID required: true schema: type: string format: uuid example: 550e8400-e29b-41d4-a716-446655440000 responses: BadRequest: description: Invalid request parameters content: application/json: schema: $ref: '#/components/schemas/Error' example: error: code: INVALID_INPUT message: Invalid request parameters request_id: 550e8400-e29b-41d4-a716-446655440000 Unauthorized: description: Authentication required or token invalid content: application/json: schema: $ref: '#/components/schemas/Error' example: error: code: UNAUTHORIZED message: Authentication required request_id: 550e8400-e29b-41d4-a716-446655440000 Forbidden: description: Insufficient permissions content: application/json: schema: $ref: '#/components/schemas/Error' example: error: code: ACCESS_DENIED message: You don't have permission to perform this action request_id: 550e8400-e29b-41d4-a716-446655440000 NotFound: description: Resource not found content: application/json: schema: $ref: '#/components/schemas/Error' example: error: code: NOT_FOUND message: Resource not found request_id: 550e8400-e29b-41d4-a716-446655440000 UnprocessableEntity: description: | The request is well-formed but cannot be processed as sent. On the operations that accept `Idempotency-Key` this is the key-reuse case: the key was first seen with a different request payload, so replaying the stored outcome would answer a question the caller did not ask. content: application/json: schema: $ref: '#/components/schemas/Error' example: error: code: IDEMPOTENCY_KEY_REUSED message: >- This Idempotency-Key was already used with a different request payload request_id: 550e8400-e29b-41d4-a716-446655440000 schemas: Error: type: object required: - error properties: error: type: object required: - code - message - request_id properties: code: type: string description: Error code identifying the type of error example: INVALID_INPUT message: type: string description: Human-readable error message example: Invalid request parameters request_id: type: string format: uuid description: Request ID for debugging example: 550e8400-e29b-41d4-a716-446655440000 securitySchemes: SignatureAuth: type: apiKey in: header name: Authorization description: > Request signing with an access key issued to a service account. An HMAC-SHA256 over a canonical form of the request, close to AWS SigV4. The `basaltic` CLI signs for you. Send `Authorization`, `X-Date` (UTC, `YYYYMMDDTHHMMSSZ`) and `X-Nonce` (random per request); add `X-Content-Sha256` to bind a body, and `X-Amz-Security-Token` when using temporary credentials. ``` Authorization: BASALTIC-HMAC-SHA256 Credential=///basaltic/basaltic_request, SignedHeaders=host;x-date;x-nonce, Signature= ``` `` is the region code you are calling, or `global` for the global services. A signature is valid for 5 minutes from `X-Date`, and mutating requests are replay-guarded on the nonce. **Full signing procedure, including a working implementation:** https://docs.basaltic.sh/authentication ## Rate limits There is no global request budget. A limit applies only where an operation documents a `429`, and that operation says what it counts. Those responses carry `X-RateLimit-Limit`, `X-RateLimit-Remaining`, `X-RateLimit-Reset` and, on a `429`, `Retry-After` — read them rather than hard-coding a number. Retrying before `Retry-After` is refused and extends the window. Everything else is bounded by quota, not by request rate. ````