ECS 3.6.1 Administration Guide

NOTE When the storage pool reaches 90% of its total capacity, it does not accept write requests and it becomes a read-only system. A storage pool must have a minimum of four nodes and must have three or more nodes with more than 10% free capacity in order to allow writes. This reserved space is required to ensure that ECS does not run out of space while persisting system metadata. If this criteria is not met, the write will fail. The ability of a storage pool to accept writes does not affect the ability of other pools to accept writes. For example, if you have a load balancer that detects a failed write, the load balancer can redirect the write to another VDC.