Optimize Index Diff & Self-bundle Updates

Cassandra doesn’t handle row deletions very efficiently. The more rows are deleted, the larger number of tombstones and the longer it takes to do lookups from the same partition.

Most of the deletions Dovecot does are index diff & self-bundle updates.

Each Dovecot Backend server always writes only a single such object per folder, which allows storing them with (user, folder, host) primary key and updating the rows on changes, instead of inserting & deleting the rows.

The diff-table parameter enables this behavior.

Diff-table requires these additional tables:

user_index_diff_objects
user_mailbox_index_diff_objects

Creation of these tables (and the corresponding dictmap settings required) are described See Simple Mapping. and See Scality sproxyd dict map respectively.