WAFL external cache initiates the cache rewarming process during a reboot or a takeover and giveback. The process keeps the cache in Flash Cache and Flash Cache 2 modules "warm."
When a storage system powers down, the WAFL external cache takes a snapshot of the data in Flash Cache and Flash Cache 2 modules. When the system powers up, it uses the snapshot to rebuild the cache. After the process completes, the system can read data from the cache.
In an HA configuration, cache rewarming is more successful when minimal changes are made to data during takeover and giveback. When you initiate takeover and giveback, the takeover partner maintains a log of data written to the down partner's storage. If there are changes to a large amount of the data that is stored in the cache, then the cache rewarming process has more data to rewarm when the node comes back online. As a result, the cache may require additional warming time.