+@code{nnfolder-directory}. This only works if you use long file names,
+though.
+
+@node Comparing Mail Backends
+@subsubsection Comparing Mail Backends
+
+First, just for terminology, the @dfn{backend} 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 backend 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 @code{nntp} or
+@code{nnspool} backends, 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 backend 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:
+
+@table @code
+@item nnmbox
+
+UNIX systems have historically had a single, very common, and well-
+defined format. All messages arrive in a single @dfn{spool file}, and
+they are delineated by a line whose regular expression matches
+@samp{^From_}. (My notational use of @samp{_} is to indicate a space,
+to make it clear in this instance that this is not the RFC-specified
+@samp{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 backend, 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.
+
+@item nnbabyl
+
+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 by Stallman.
+
+Both of the above forms leave your mail in a single file on your
+filesystem, and they must parse that entire file each time you take a
+look at your mail.
+
+@item nnml
+
+@code{nnml} is the backend which smells the most as though you were
+actually operating with an @code{nnspool}-accessed Usenet system. (In
+fact, I believe @code{nnml} actually derived from @code{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) @file{/var/lib/news/active},
+or what is returned via the @samp{NNTP LIST} verb) and also creates
+@dfn{overview} files for efficient group entry, as has been defined for
+@sc{nntp} servers for some years now. It is slower in mail-splitting,
+due to the creation of lots of files, updates to the @code{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.
+
+@code{nnml} costs @dfn{inodes} in a big way; that is, it soaks up the
+resource which defines available places in the filesystem to put new
+files. Sysadmins take a dim view of heavy inode occupation within
+tight, shared filesystems. But if you live on a personal machine where
+the filesystem is your own and space is not at a premium, @code{nnml}
+wins big.
+
+It is also problematic using this backend if you are living in a
+FAT16-based Windows world, since much space will be wasted on all these
+tiny files.
+
+@item nnmh
+
+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 -- @code{nnmh}
+is considered to be semantically equivalent to ``@code{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.
+
+@item nnfolder
+
+Basically the effect of @code{nnfolder} is @code{nnmbox} (the first
+method described above) on a per-group basis. That is, @code{nnmbox}
+itself puts *all* one's mail in one file; @code{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 @code{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, @code{nnfolder} is not the best choice, but if you receive
+only a moderate amount of mail, @code{nnfolder} is probably the most
+friendly mail backend all over.
+
+@end table
+