StorageGRID reserves space on storage volume 0 of each Storage Node for the Cassandra database. This database stores three copies of all object metadata as well as certain configuration data. To ensure that adequate space remains available for essential Cassandra operations, you must monitor the metadata attributes for each Storage Node and add new Storage Nodes as required.
The total space reserved on each Storage Node for metadata is known as Metadata Reserved Space, or CAWM. CAWM is subdivided into the space available for object metadata (the Metadata Allowed Space, or CEMS) and the space required for essential Cassandra operations, such as compaction and repair.
If object metadata uses more than 100% of the Metadata Allowed Space, Cassandra operations cannot run efficiently and errors will occur. For this reason, you must closely monitor how much object metadata space has been used.
The Metadata Used Space (Percent) attribute, or CDLP, measures how full the Metadata Allowed Space is. When the Metadata Used Space (Percent) reaches certain thresholds, the CDLP alarm is triggered, as follows:
Note that when the Metadata Used Space (Percent) attribute reaches 70% (that is, when the Metadata Allowed Space becomes 70% full), the CDLP alarm is triggered as a minor alarm. You should add new Storage Nodes in an expansion procedure as soon as possible. When you add the new nodes, the system automatically rebalances object metadata across all Storage Nodes, and the alarms clear.