func (v *mockFilesystemAccessor) FilesystemParams(filesystems []names.FilesystemTag) ([]params.FilesystemParamsResult, error) {
	var result []params.FilesystemParamsResult
	for _, tag := range filesystems {
		if _, ok := v.provisionedFilesystems[tag.String()]; ok {
			result = append(result, params.FilesystemParamsResult{
				Error: &params.Error{Message: "already provisioned"},
			})
		} else {
			filesystemParams := params.FilesystemParams{
				FilesystemTag: tag.String(),
				Size:          1024,
				Provider:      "dummy",
				Tags: map[string]string{
					"very": "fancy",
				},
			}
			if _, ok := names.FilesystemMachine(tag); ok {
				// place all volume-backed filesystems on machine-scoped
				// volumes with the same ID as the filesystem.
				filesystemParams.VolumeTag = names.NewVolumeTag(tag.Id()).String()
			}
			result = append(result, params.FilesystemParamsResult{Result: filesystemParams})
		}
	}
	return result, nil
}

// FilesystemParams returns the parameters for creating or destroying the
// given filesystem.
func FilesystemParams(
	f state.Filesystem,
	storageInstance state.StorageInstance,
	modelUUID, controllerUUID string,
	environConfig *config.Config,
	poolManager poolmanager.PoolManager,
	registry storage.ProviderRegistry,
) (params.FilesystemParams, error) {

	var pool string
	var size uint64
	if stateFilesystemParams, ok := f.Params(); ok {
		pool = stateFilesystemParams.Pool
		size = stateFilesystemParams.Size
	} else {
		filesystemInfo, err := f.Info()
		if err != nil {
			return params.FilesystemParams{}, errors.Trace(err)
		}
		pool = filesystemInfo.Pool
		size = filesystemInfo.Size
	}

	filesystemTags, err := storageTags(storageInstance, modelUUID, controllerUUID, environConfig)
	if err != nil {
		return params.FilesystemParams{}, errors.Annotate(err, "computing storage tags")
	}

	providerType, cfg, err := StoragePoolConfig(pool, poolManager, registry)
	if err != nil {
		return params.FilesystemParams{}, errors.Trace(err)
	}
	result := params.FilesystemParams{
		f.Tag().String(),
		"", // volume tag
		size,
		string(providerType),
		cfg.Attrs(),
		filesystemTags,
		nil, // attachment params set by the caller
	}

	volumeTag, err := f.Volume()
	if err == nil {
		result.VolumeTag = volumeTag.String()
	} else if err != state.ErrNoBackingVolume {
		return params.FilesystemParams{}, errors.Trace(err)
	}

	return result, nil
}

func (v *mockFilesystemAccessor) FilesystemParams(filesystems []names.FilesystemTag) ([]params.FilesystemParamsResult, error) {
	results := make([]params.FilesystemParamsResult, len(filesystems))
	for i, tag := range filesystems {
		// Parameters are returned regardless of whether the filesystem
		// exists; this is to support destruction.
		filesystemParams := params.FilesystemParams{
			FilesystemTag: tag.String(),
			Size:          1024,
			Provider:      "dummy",
			Tags: map[string]string{
				"very": "fancy",
			},
		}
		if _, ok := names.FilesystemMachine(tag); ok {
			// place all volume-backed filesystems on machine-scoped
			// volumes with the same ID as the filesystem.
			filesystemParams.VolumeTag = names.NewVolumeTag(tag.Id()).String()
		}
		results[i] = params.FilesystemParamsResult{Result: filesystemParams}
	}
	return results, nil
}