MIX (Email)

MIX is a high-performance, indexed, on-disk email storage system that is designed for use with the IMAP protocol. MIX was designed by Mark Crispin, the author of the IMAP protocol. Server support for it has been included in releases of UW IMAP since 2006,Panda IMAP, and Messaging Architects Netmail. MIX is also supported directly by the Alpine e-mail client.

MIX mailboxes are directories containing several types of files, including a metadata file, an index file, a dynamic status data file, a threading/sorting cache file, and a collection of files containing message content. MIX mailboxes can also contain subordinate mailboxes, which are implemented as sub directories within the MIX directory.

The MIX format was designed with an emphasis on very high scalability, reliability, and performance, while efficiently supporting modern features of the IMAP protocol. MIX has been used successfully with mailboxes of 750,000 messages.

The base level MIX format has four files: a metadata file, an index file, a status file, and some set of message data files. The metadata file contains base-level data applicable to the entire mailbox; i.e., the UID validity, last assigned UID, and list of keywords. The index file contains pointers to each unexpunged message in the message data files, along with flags, size, and IMAP internaldate data. The status file contains per-message flags and keywords.

All these files may be hidden files in a directory (with the directory name being the name of the mailbox). Thus a directory with gigabytes of mail in it may appear to be empty if examined with tools that don't show hidden files. This is a common source of confusion for system administrators encountering MIX for the first time.

By design, it is possible to recover the mailbox into a usable state if any of these files is lost or corrupted. For example, it is possible to rebuild the index file by reading each of the data files, with no consequence other than the possible "unexpunging" of an expunged message that had not yet had its space recovered.

Another important part of the MIX design is that no file is modified unless the data specific to that file is altered; thus a flag change alters the status file but not the metadata or index files. This reduces the impact of any system event that corrupts a file write in progress.

Each file also has a "modification sequence" which is incremented each time the file is changed. When a MIX implementation updates from a file, if the modification sequence is unchanged it closes the file at once without reading it further. In addition, each status file entry also has a modification sequence, which permits lossless synchronization of multiple consumer message flag/keyword updates/

...
Wikipedia