The IMAP server is heavily loaded during peak hours because the ACL tracks which users can access which mailboxes.
Write performance is critical, as folders are created and deleted continuously.
The mailboxes file was being stored on a conventional RAID system using rotating disks. This approach had been maintained and scaled while the user base grew over twenty-fold. The actual mailboxes file is relatively small, measuring megabytes in size. However, each active user requires storage space related to this file. The amount of storage depends on the number of users and the amount of folder activity.