Cyrus IMAP uses directory hashing for per-user state information,
including quotas (with
quota_db: quotalegacy), sieve, subscriptions
and seen information. Sites with lots of users may wish to hash their
mail spool, too. Cyrus provides mechanisms for this. This pair of
settings for imapd.conf(5) control hashing behavior:
If enabled, the partitions will also be hashed, in addition to the hashing done on configuration directories. This is recommended if one partition has a very bushy mailbox tree.
If enabled, uses an improved directory hashing scheme which hashes on the entire username instead of using just the first letter as the hash. This changes hash algorithm used for quota and user directories and if hashimapspool is enabled, the entire mail spool.
Note that this option CANNOT be changed on a live system. The server must be quiesced and then the directories moved with the rehash utility.
Regardless of those settings, the per-user state information will always be hashed.