You must monitor the value of the Total Usable Space (STAS) attribute for each Storage Node to ensure that the node has enough space for new object data.
About this task
STAS is calculated by adding together the available space on all object stores within the Storage Node.

Note: If the value of STAS for a Storage Node falls below the value of the Storage Volume Hard Read-Only Watermark, the Storage Node becomes read-only and can no longer store new objects. You should add storage volumes or Storage Nodes before this happens.