1. A method of implementing a selective read-only mode in a storage system with a transaction journal, comprising: monitoring a condition that affects data stored in a transaction journal corresponding to a portion of a storage system, wherein the transaction journal records in a persistent storage transactions that modify user data stored in the portion; detecting the condition; entering a read-only mode for the portion of the storage system; enforcing a policy of processing write requests and read requests to the storage system, comprising: processing the write requests without modifying user data on the portion; and processing the read requests, including requests for user data stored on the portion.