1. A method of entering a read-only mode for a portion of a storage system, comprising: marking as unwritable a transaction journal associated with a portion of a storage system entering a read-only mode; backing up the transaction journal without writing to the portion; continuing to process, by one or more computer processors, unresolved transactions affecting the portion by marking as commit deferred, without writing to the transaction journal, those transactions for which a commit message is received after entering the read-only mode; and processing, by one or more computer processors, those transactions marked commit deferred, after leaving the read-only mode for the portion, by marking each of those transactions as committed in the transaction journal, and sending committed messages to other affected portions of the storage system.