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First, just for terminology, the back end is the common word for a low-level access method--a transport, if you will, by which something is acquired. The sense is that one's mail has to come from somewhere, and so selection of a suitable back end is required in order to get that mail within spitting distance of Gnus.

The same concept exists for Usenet itself: Though access to articles is typically done by NNTP these days, once upon a midnight dreary, everyone in the world got at Usenet by running a reader on the machine where the articles lay (the machine which today we call an NNTP server), and access was by the reader stepping into the articles' directory spool area directly. One can still select between either the nntp or nnspool back ends, to select between these methods, if one happens actually to live on the server (or can see its spool directly, anyway, via NFS).

The goal in selecting a mail back end is to pick one which simultaneously represents a suitable way of dealing with the original format plus leaving mail in a form that is convenient to use in the future. Here are some high and low points on each:


UNIX systems have historically had a single, very common, and well- defined format. All messages arrive in a single spool file, and they are delineated by a line whose regular expression matches `^From_'. (My notational use of `_' is to indicate a space, to make it clear in this instance that this is not the RFC-specified `From:' header.) Because Emacs and therefore Gnus emanate historically from the Unix environment, it is simplest if one does not mess a great deal with the original mailbox format, so if one chooses this back end, Gnus' primary activity in getting mail from the real spool area to Gnus' preferred directory is simply to copy it, with no (appreciable) format change in the process. It is the "dumbest" way to move mail into availability in the Gnus environment. This makes it fast to move into place, but slow to parse, when Gnus has to look at what's where.


Once upon a time, there was the DEC-10 and DEC-20, running operating systems called TOPS and related things, and the usual (only?) mail reading environment was a thing called Babyl. I don't know what format was used for mail landing on the system, but Babyl had its own internal format to which mail was converted, primarily involving creating a spool-file-like entity with a scheme for inserting Babyl-specific headers and status bits above the top of each message in the file. Rmail was Emacs' first mail reader, it was written by Richard Stallman, and Stallman came out of that TOPS/Babyl environment, so he wrote Rmail to understand the mail files folks already had in existence. Gnus (and VM, for that matter) continue to support this format because it's perceived as having some good qualities in those mailer-specific headers/status bits stuff. Rmail itself still exists as well, of course, and is still maintained within Emacs. Since Emacs 23, it uses standard mbox format rather than Babyl.

Both of the above forms leave your mail in a single file on your file system, and they must parse that entire file each time you take a look at your mail.


nnml is the back end which smells the most as though you were actually operating with an nnspool-accessed Usenet system. (In fact, I believe nnml actually derived from nnspool code, lo these years ago.) One's mail is taken from the original spool file, and is then cut up into individual message files, 1:1. It maintains a Usenet-style active file (analogous to what one finds in an INN- or CNews-based news system in (for instance) `/var/lib/news/active', or what is returned via the `NNTP LIST' verb) and also creates overview files for efficient group entry, as has been defined for NNTP servers for some years now. It is slower in mail-splitting, due to the creation of lots of files, updates to the nnml active file, and additions to overview files on a per-message basis, but it is extremely fast on access because of what amounts to the indexing support provided by the active file and overviews.

nnml costs inodes in a big way; that is, it soaks up the resource which defines available places in the file system to put new files. Sysadmins take a dim view of heavy inode occupation within tight, shared file systems. But if you live on a personal machine where the file system is your own and space is not at a premium, nnml wins big.

It is also problematic using this back end if you are living in a FAT16-based Windows world, since much space will be wasted on all these tiny files.


The Rand MH mail-reading system has been around UNIX systems for a very long time; it operates by splitting one's spool file of messages into individual files, but with little or no indexing support---nnmh is considered to be semantically equivalent to "nnml without active file or overviews". This is arguably the worst choice, because one gets the slowness of individual file creation married to the slowness of access parsing when learning what's new in one's groups.


Basically the effect of nnfolder is nnmbox (the first method described above) on a per-group basis. That is, nnmbox itself puts all one's mail in one file; nnfolder provides a little bit of optimization to this so that each of one's mail groups has a Unix mail box file. It's faster than nnmbox because each group can be parsed separately, and still provides the simple Unix mail box format requiring minimal effort in moving the mail around. In addition, it maintains an "active" file making it much faster for Gnus to figure out how many messages there are in each separate group.

If you have groups that are expected to have a massive amount of messages, nnfolder is not the best choice, but if you receive only a moderate amount of mail, nnfolder is probably the most friendly mail back end all over.


For configuring expiry and other things, nnmaildir uses incompatible group parameters, slightly different from those of other mail back ends.

nnmaildir is largely similar to nnml, with some notable differences. Each message is stored in a separate file, but the filename is unrelated to the article number in Gnus. nnmaildir also stores the equivalent of nnml's overview files in one file per article, so it uses about twice as many inodes as nnml. (Use df -i to see how plentiful your inode supply is.) If this slows you down or takes up very much space, consider switching to ReiserFS or another non-block-structured file system.

Since maildirs don't require locking for delivery, the maildirs you use as groups can also be the maildirs your mail is directly delivered to. This means you can skip Gnus' mail splitting if your mail is already organized into different mailboxes during delivery. A directory entry in mail-sources would have a similar effect, but would require one set of mailboxes for spooling deliveries (in mbox format, thus damaging message bodies), and another set to be used as groups (in whatever format you like). A maildir has a built-in spool, in the new/ subdirectory. Beware that currently, mail moved from new/ to cur/ instead of via mail splitting will not undergo treatment such as duplicate checking.

nnmaildir stores article marks for a given group in the corresponding maildir, in a way designed so that it's easy to manipulate them from outside Gnus. You can tar up a maildir, unpack it somewhere else, and still have your marks. nnml also stores marks, but it's not as easy to work with them from outside Gnus as with nnmaildir.

nnmaildir uses a significant amount of memory to speed things up. (It keeps in memory some of the things that nnml stores in files and that nnmh repeatedly parses out of message files.) If this is a problem for you, you can set the nov-cache-size group parameter to something small (0 would probably not work, but 1 probably would) to make it use less memory. This caching will probably be removed in the future.

Startup is likely to be slower with nnmaildir than with other back ends. Everything else is likely to be faster, depending in part on your file system.

nnmaildir does not use nnoo, so you cannot use nnoo to write an nnmaildir-derived back end.

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