Explanation/Reference:
Metrics are a series of measurements that are computed and retained in memory for an interval of time, and stored on a disk for a more permanent history.
On the storage servers, the CELLSRV process provides the majority of Oracle Exadata storage services and is the primary storage software component. One of its functions is to process, collect, and store metrics. The Management Server (MS) process receives the metrics data from CELLSRV, keeps a subset of metrics in memory, and writes to an internal disk-based repository hourly.
References: http://www.oracle.com/technetwork/articles/servers-storage-admin/monitor-exadata-em12-
2291964.html