Ingesters rolling updates

Cortex ingesters are semi-stateful. A running ingester holds several hours of time series data in memory before they're flushed to the long-term storage. When an ingester shuts down because of a rolling update or maintenance, the in-memory data must not be discarded in order to avoid any data loss.

The Cortex blocks storage requires ingesters to run with a persistent disk where the TSDB WAL and blocks are stored (eg. a StatefulSet when deployed on Kubernetes).

During a rolling update, the leaving ingester closes the open TSDBs, synchronizes the data to disk (fsync), and releases the disk resources. The new ingester, which is expected to reuse the same disk as the leaving one, will replay the TSDB WAL on startup to load back into memory the time series that have not been compacted into a block yet.