prefix setting now passed to all borg commands that support it.

refined documentation of archive and prefix settings.

doc/configuring.rst

@@ -98,22 +98,6 @@ or Emborg's program name ('prog_name'). An example of this is shown in both
 In this way you gain access to *Borg* settings. *archive* shows and example of 
 that.
 
-It is generally better to specify *prefix* rather than *archive*, though you can 
-specify both if you wish.  If *archive* is not specified and *prefix* is, then 
-*archive* is created by adding '{{now}}' to *prefix*. If *archive* is specified 
-but *prefix* is not, then *archive* is used as given and *prefix* is not passed 
-to the *prune* command. In this case you should not have multiple configurations 
-backup into the same repository.
-
-For example, if *prefix* is '{config_name}-' and *archive* is not given then 
-*archive* becomes '{config_name}-{{now}}'.  If neither *archive* or *prefix* is 
-specified, then '{config_name}-{{now}}' is used for *archive* and 
-'{config_name}-' is used for *prefix*.  One would specify both *archive* and 
-*prefix* when you wanted to use a different format for *now*. For example::
-
-    archive = '{config_name}-{{now:%Y%m%d}}'
-    prefix = '{config_name}-'
-
 
 Includes
 --------

doc/installing.rst

@@ -70,13 +70,63 @@ repository.  For a local repository you would use something like this::
 
     repository = '/mnt/backups/{host_name}-{user_name}-{config_name}'
 
+This assumes that */mnt/backups* contains many independent respositories.
+Borg allows you to make a single repository the target of multiple backup 
+configurations, and in this way you can further benefit from its ability to 
+de-duplicate files.  In this case you might want to use a less granular name for 
+you respository.
 
-**archive**
+**archive** and **prefix**
 
-A template that is used when creating the archive name. A typical value might 
-be::
+*archive* is a template that specifies the name of each archive.  A typical 
+value might be::
 
-    archive = '{host_name}-{{now}}'
+    archive = '{config_name}-{{now}}'
+
+This template consists of a leading part that is fixed ('{config_name}-') and 
+a trailing part that varies on each archive ('{{now}}', which is replaced by 
+a datestamp). The leading fixed part is referred to as the *prefix* and can be 
+given separately::
+
+    archive = '{config_name}-{{now}}'
+    prefix = '{config_name}-'
+
+This is helpful when multiple configurations backup to the same repository. In 
+this case *prefix* is assumed to be unique between the configurations. It allows 
+certain commands to filter out archives that belong to other configurations.  
+Specifically the *check*, *delete*, *info*, *list*, *mount*, and *prune* 
+commands all use *prefix*.
+
+When sharing a repository between multiple backup configurations, it is 
+important that all prefixes be unique. Be careful of one prefix that is a prefix 
+of another. For example, prefixes of *root* and *root2* would be bad because 
+*root* is a prefix of *root2*.  In the examples given, *prefix* ends with '-' to 
+reduce the risk that one prefix could be a prefix of another.
+
+If you do not specify either *archive* or *prefix*, they you get the following 
+defaults::
+
+    archive = '{config_name}-{{now}}'
+    prefix = '{config_name}-'
+
+If you only specify *prefix*, then *archive* becomes::
+
+    archive = '<prefix>{{now}}'
+
+If you only specify *archive*, then *prefix* remains unset. This is only 
+suitable when there is only one backup configuration using a repository.
+
+If you want *prefix* and want to customize *now*, you should give both *prefix* 
+and *archive*. For example, you can reduce the length of the timestamp using::
+
+    archive = '{host_name}-{{now:%Y%m%d}}'
+    prefix = '{host_name}-'
+
+In this example the host name was used as the prefix rather than the 
+configuration name. When specifying both the *prefix* and the *archive*, the 
+leading part of *archive* should match *prefix*.  Be aware that by including 
+only the date in the archive name rather than the full timestamp, you are 
+limiting yourself to one archive per day.
 
 
 **encryption**

emborg/preferences.py

@@ -158,7 +158,7 @@
         desc = 'set umask to M (local and remote, default: 0077)'
     ),
     prefix = dict(
-        cmds = 'prune',
+        cmds = 'check delete info list mount prune',
         arg = 'PREFIX',
         desc = 'only consider archive names starting with this prefix'
     ),