func createStorageDetails(st storageAccess, si state.StorageInstance) (*params.StorageDetails, error) {
	// Get information from underlying volume or filesystem.
	var persistent bool
	var statusEntity state.StatusGetter
	if si.Kind() != state.StorageKindBlock {
		// TODO(axw) when we support persistent filesystems,
		// e.g. CephFS, we'll need to do set "persistent"
		// here too.
		filesystem, err := st.StorageInstanceFilesystem(si.StorageTag())
		if err != nil {
			return nil, errors.Trace(err)
		}
		statusEntity = filesystem
	} else {
		volume, err := st.StorageInstanceVolume(si.StorageTag())
		if err != nil {
			return nil, errors.Trace(err)
		}
		if info, err := volume.Info(); err == nil {
			persistent = info.Persistent
		}
		statusEntity = volume
	}
	status, err := statusEntity.Status()
	if err != nil {
		return nil, errors.Trace(err)
	}

	// Get unit storage attachments.
	var storageAttachmentDetails map[string]params.StorageAttachmentDetails
	storageAttachments, err := st.StorageAttachments(si.StorageTag())
	if err != nil {
		return nil, errors.Trace(err)
	}
	if len(storageAttachments) > 0 {
		storageAttachmentDetails = make(map[string]params.StorageAttachmentDetails)
		for _, a := range storageAttachments {
			machineTag, location, err := storageAttachmentInfo(st, a)
			if err != nil {
				return nil, errors.Trace(err)
			}
			details := params.StorageAttachmentDetails{
				a.StorageInstance().String(),
				a.Unit().String(),
				machineTag.String(),
				location,
			}
			storageAttachmentDetails[a.Unit().String()] = details
		}
	}

	return &params.StorageDetails{
		StorageTag:  si.Tag().String(),
		OwnerTag:    si.Owner().String(),
		Kind:        params.StorageKind(si.Kind()),
		Status:      common.EntityStatusFromState(status),
		Persistent:  persistent,
		Attachments: storageAttachmentDetails,
	}, nil
}

// populateStatusFromGetter creates status information for machines, units.
func populateStatusFromGetter(agent *params.AgentStatus, getter state.StatusGetter) {
	statusInfo, err := getter.Status()
	agent.Err = err
	agent.Status = params.Status(statusInfo.Status)
	agent.Info = statusInfo.Message
	agent.Data = filterStatusData(statusInfo.Data)
	agent.Since = statusInfo.Since
}

// makeStatusForEntity creates status information for machines, units.
func makeStatusForEntity(agent *api.AgentStatus, getter state.StatusGetter) {
	statusInfo, err := getter.Status()
	agent.Err = err
	agent.Status = params.Status(statusInfo.Status)
	agent.Info = statusInfo.Message
	agent.Data = filterStatusData(statusInfo.Data)
	agent.Since = statusInfo.Since
}