document snapshot usage #2178

docs/faq.rst

@@ -429,7 +429,7 @@ BORG_FILES_CACHE_TTL to at least 26 (or maybe even a small multiple of that),
 it would be much faster.
 
 Another possible reason is that files don't always have the same path, for
-example if you mount a filesystem without stable mount points for each backup.
+example if you mount a filesystem without stable mount points for each backup or if you are running the backup from a filesystem snapshot whose name is not stable.
 If the directory where you mount a filesystem is different every time,
 |project_name| assume they are different files.
 

src/borg/archiver.py

@@ -2122,6 +2122,11 @@ class Archiver:
         potentially decreases reliability of change detection, while avoiding always reading
         all files on these file systems.
 
+        The mount points of filesystems or filesystem snapshots should be the same for every
+        creation of a new archive to ensure fast operation. This is because the file cache that
+        is used to determine changed files quickly uses absolute filenames.
+        If this is not possible, consider creating a bind mount to a stable location.
+
         See the output of the "borg help patterns" command for more help on exclude patterns.
         See the output of the "borg help placeholders" command for more help on placeholders.