Spec Lite: Limit number of allowed replicas per share


Currently, Manila allows the creation of unlimited share replicas of a given share. It is a problem for both DHSS=True and DHSS=False modes. It allows the user to deplete the storage resources and have troubles depending on the configured backend. There is also a problem when it comes to the user visibility side on what backend do or do not support. Some backends do not support more than a limited number of share replicas, and it is not noticeable until the backend asynchronously denies the operation.


We should let the administrator specify the max number of replicas that a given share is able to have according to the backend capacity. In this way, the API would be able to check the number of existent share replicas during a new replica creation request, and be able to fail faster and synchronously. The proposed solution is to add a new extra spec to the share types called max_replicas_per_share. Then, when creating a share replica, Manila will check the max_replicas_per_share value of the source share’s share type, and will verify the number of existent replicas of the source share. If the number of existent share replicas has already reached the max_replicas_per_share value, the request to create a new replica will be denied. While creating the new extra-spec for the share type the administrator must specify an integer value. A database migration will be created in order to add the new share type extra spec for all existent share types. In the database migration, a default of two replicas will be set due to some backends capacity. All administrators will need to update the share type according to their backend capacity, and will set the extra spec according to the number of replicas that shares created under the given share type are able to have. When creating a new share type, if the user does not specify the new extra spec, the service will automatically set this value to 2 and in case the administrators need to change the value, they will need to update the extra spec. The administrator will also be able to provide a new backend property called supported_replicas_per_share for each backend. The scheduler will be improved in order to filter backends considering this backend property. This solution can be combined with improvements, which consists of defining a project quota for share replicas using the existent quota system, and allow the administrator to set a default value for the max_replicas_per_share extra spec. So at the end, the whole solution would have a way to avoid this bug, alongside with an improvement that will help administrators to manage their resources. The solution for the bug will require the administrator intervention, since the database migration script will update all the share types with a default value, and it may block users while creating new share replicas after migrating the database. It is not currently possible to set per-share replica limits as proposed in this specification, but the manila project quota controls can be applied to share replicas since Ussuri release.

  • REST API Impact.
    • When creating a share replica, if the share has already reached the number of allowed share replicas, the API will return 403 Forbidden with the following message: “The share with the id ‘share_id’ already reached the limit of replicas according to its share type”.

    • The API will expect an integer value while setting the max_replicas_per_share extra spec, otherwise it will return 400 BadRequest with the following message: “The specified max replicas value ‘non_integer_value’ must be an integer and greater than or equal to 2.”.

  • Documentation Impact
    • Admin guide

    • API Reference


Include in Ussuri release.