Man directory layout -- why the subdirectories?

Question

Man expects the man directories listed in $MANPATH or $(manpath) to be split by section into directories named "man$section". This duplicates the section information that is already available in the suffix of the manpage. (e.g. for ls.1.gz, the .1 info gets duplicated in man1/). Why not skip the middle man-directories and make the manpath directories flat when flat seems good enough for $PATH directories?

Answer 1

It doesn't duplicate the information: you can have more suffixes in a given directory than the plain ".1" or ".3", e.g., (depending on the platform) letters following the numbers. For example, Debian follows the ".3" with a an application suffix such as "pm" for Perl modules.

Here is (part) of the listing from /usr/share/man/man1, to illustrate:

-rw-r--r--   1 root          592 Apr 17 2012    411toppm.1.gz
-rw-r--r--   1 root         3827 Tue 15:21:13   CA.pl.1ssl.gz
lrwxrwxrwx   1 root           17 Feb 19 2012    GET.1p.gz -> lwp-request.1p.gz
lrwxrwxrwx   1 root           17 Feb 19 2012    HEAD.1p.gz -> lwp-request.1p.gz
lrwxrwxrwx   1 root           17 Feb 19 2012    POST.1p.gz -> lwp-request.1p.gz
-rw-r--r--   1 root         2490 Aug 29 2011    SOAPsh.1p.gz
-rw-r--r--   1 root         2428 Aug 29 2011    XMLRPCsh.1p.gz
-rw-r--r--   1 root         5112 Apr  5 2012    alien.1p.gz 
-rw-r--r--   1 root         3130 Oct 26 2012    apt-show-versions.1p.gz
-rw-r--r--   1 root         4011 Tue 15:21:13   asn1parse.1ssl.gz
-rw-r--r--   1 root         2847 Tue 15:21:13   c_rehash.1ssl.gz
-rw-r--r--   1 root         9796 Tue 15:21:13   ca.1ssl.gz
-rw-r--r--   1 root         6410 Tue 15:21:13   ciphers.1ssl.gz
-rw-r--r--   1 root         8419 Tue 15:21:13   cms.1ssl.gz
-rw-r--r--   1 root         6394 Jun 26 2012    cpanm.1p.gz
-rw-r--r--   1 root         2631 Tue 15:21:13   crl.1ssl.gz
-rw-r--r--   1 root         2636 Tue 15:21:13   crl2pkcs7.1ssl.gz
-rw-r--r--   1 root         2272 Jun 19 2014    dbilogstrip.1p.gz
-rw-r--r--   1 root         3255 Jun 19 2014    dbiprof.1p.gz

Additionally, the various directories are split up because in systems using cat directories, the filenames would be (usually) duplicated.

And finally - there's a split-up to keep directory size (relatively) small and improve performance.

Answer 2

At this point I'd say the main reason is backwards compatibility — the directory split was there right from the start, in V4 (that's the fourth release of UNIX, not SVR4). Back then there could have been any number of reasons: avoiding having to handle many files in a single directory, thinking of the manual pages as parts of a book...