Misc docs updates (#1544)

mkdocs/docs/faq/recommendations_and_warnings.md

@@ -9,9 +9,9 @@
 * VM images: For the same reasons as databases. VM images are accessed
   very aggressively and mergerfs will introduce a lot of extra latency.
 * As replacement for RAID: mergerfs is just for pooling branches. If
-  you need that kind of device performance aggregation or high
-  availability you should stick with RAID. However, it is fine to put
-  a filesystem which is on a RAID setup in mergerfs.
+  you need device performance aggregation or high availability you
+  should stick with RAID. However, it is fine to put a filesystem
+  which is on a RAID setup in mergerfs.
   
 **However, if using [passthrough](../config/passthrough.md) the above
 situations are less likely to be a concern. Best to do testing for

mkdocs/docs/faq/why_isnt_it_working.md

@@ -230,13 +230,29 @@ the software's author and request proper handling of `EXDEV` errors.
 * [Moving files and directories fails with Samba](../known_issues_bugs.md#moving-files-and-directories-fails-with-samba)
 
 
-## Why is rtorrent failing with ENODEV (No such device)?
+## Why is software needing mmap failing?
 
-Be sure to set
-[cache.files=partial|full|auto-full|per-process](../config/cache.md)
-or use Linux kernel v6.6 or above. rtorrent and some other
-applications use [mmap](http://linux.die.net/man/2/mmap) to read and
-write to files and offer no fallback to traditional methods.
+See [Known Issues and Bugs#3rd Party
+Software](../known_issues_bugs.md#3rd-party-software) and
+[QuickStart](../quickstart.md).
+
+
+## Why don't quick scans fail to pick up new media?
+
+Media consumption software like Plex, Jellyfin, Emby, Airsonic,
+Navidrome, etc. will use timestamps of directories to see if it should
+be scanned. Be sure to set `func.getattr=newest` to catch that.
+
+A full scan would still pick up all media and for those concerned
+about device activity it is recommened to disable quick scans and
+leave periodic full scans so the activity is managed. Or to configure
+your software which manages downloads (such as *Arr family of
+software) to trigger specific scans when files are added to the pool.
+
+Additional reading:
+
+* [Does inotify and fanotify
+work?](compatibility_and_integration.md#does-inotify-and-fanotify-work)
 
 
 ## Why don't I see mergerfs options in mount command or /proc/mounts?

mkdocs/docs/index.md

@@ -5,7 +5,10 @@
 [union filesystem](https://en.wikipedia.org/wiki/Union_mount) geared
 towards simplifying storage and management of files across numerous
 commodity storage devices. It is similar to [**mhddfs**, **unionfs**,
-**aufs**, **DrivePool**, etc.](project_comparisons.md)
+**aufs**, **DrivePool**, etc.](project_comparisons.md) Allowing for
+the aggregation of storage without the more rigid requirements and
+failure conditions of [RAID and other
+technologies](project_comparisons.md).
 
 
 ## Features
@@ -35,7 +38,8 @@ commodity storage devices. It is similar to [**mhddfs**, **unionfs**,
 
 * Read/write overlay on top of read-only filesystem like OverlayFS
 * File whiteout
-* RAID like parity calculation (see [SnapRAID](https://www.snapraid.it))
+* RAID like redundancy (see [SnapRAID](https://www.snapraid.it) and
+  [NonRAID](https://github.com/qvr/nonraid))
 * Redundancy
 * Splitting of files across branches
 

mkdocs/docs/known_issues_bugs.md

@@ -91,36 +91,41 @@ on [page caching](config/cache.md).
 
 ## 3rd Party Software
 
-### SQLite3, Plex, Jellyfin do not work with mergerfs
-
-It does. If you're trying to put the software's config / metadata /
-database on mergerfs you can't set [cache.files=off](config/cache.md)
-(unless you use Linux v6.6 or above and
-[direct-io-allow-mmap](config/options.md) is enabled) because they are
-using **sqlite3** with **mmap** enabled and have failed to properly
-handle the situation where **mmap** may not be available.
+### Software using mmap
+
+* software using sqlite3 w/ mmap
+    * Plex (for config)
+    * Jellyfin (for config)
+    * Emby (for config)
+    * Radarr
+    * Sonarr
+    * Lidarr
+* other
+    * rtorrent
+    * qbittorrent (with 'Disk IO type = Memory mapped files')
+
+[mmap](https://en.wikipedia.org/wiki/Mmap) is a filesystem and
+operating system feature allowing files to be interacted with as if it
+was normal memory. Support for `mmap` is **required** by a number of
+projects. FUSE (and therefore mergerfs), in order to support `mmap`,
+requires [page caching](config/cache.md) to be
+enabled. IE `cache.files` must not be set to `off` (if using Linux before
+v6.6 or mergerfs before v2.41.0.)
+
+In newer versions of mergerfs and Linux `off` will work as it can now
+auto enable page caching (and therefore `mmap`) as needed.
 
 That said it is recommended that config and runtime files be stored on
 SSDs on a regular filesystem for performance reasons. See [What should
 mergerfs NOT be used
 for?](faq/recommendations_and_warnings.md#what-should-mergerfs-not-be-used-for).
 
-Other software that leverages **sqlite3** which require **mmap**
-includes Radarr, Sonarr, and Lidarr. However, many programs use
-**sqlite3** and do not require **mmap**.
-
-It is recommended that you reach out to the developers of the software
-you are having troubles with and ask them to add a fallback to regular
-file IO when **mmap** is unavailable. It is not only more compatible
-but also can be more performant in certain situations.
-
-If the issue is that quick scans do not seem to pick up media then be
-sure to set `func.getattr=newest`. That said a full scan will pick up
-all media and it will put less load on the host to use time based
-library scans or to configure downloading software to trigger a scan
-when files are added to the pool. See [Does inotify and fanotify
-work?](faq/compatibility_and_integration.md#does-inotify-and-fanotify-work)
-for more details.
+Technically **sqlite3** does not require `mmap`, and in fact the
+default is not to use it, but many programs enable the feature and
+provide no error handling when not supported. Ideally the software
+would catch the error and use traditional IO instead. Not only is it
+more compatible but could also be more performant in certain
+situations.
 
 
 ### backup software

mkdocs/docs/project_comparisons.md

@@ -1,5 +1,8 @@
 # Project Comparisons
 
+There are no solutions, only trade-offs.
+
+
 ## mhddfs
 
 * [https://romanrm.net/mhddfs](https://romanrm.net/mhddfs)

mkdocs/docs/quickstart.md

@@ -25,24 +25,22 @@ cases.
 * func.getattr=newest
 * dropcacheonclose=false
 
-In previous versions of Linux it was unable to support `mmap` if [page
-caching](config/cache.md) was disabled (ie:
-`cache.files=off`). However, it now will enable page caching if needed
-for a particular file if `mmap` is requested.
+See [Known Issues and Bugs#Software Using
+mmap](known_issues_bugs.md#software-using-mmap) for more details why
+the Linux and mergerfs version matter and its relation to `mmap`.
 
-[mmap](https://en.wikipedia.org/wiki/Mmap) is needed by certain
-software to read and write to a file. However, many software could
-work without it and fail to have proper error handling for when it is
-unavailable. Many programs that use **sqlite3** will require `mmap`
-despite [sqlite3 working perfectly
-fine](known_issues_bugs.md#sqlite3-plex-jellyfin-do-not-work-with-mergerfs)
-without it (and in some cases can be more performant with regular file
-IO.)
+**NOTE:** This "auto mmap" feature is only supported in mergerfs
+v2.41.0 and above. For older versions see below or the docs from that
+release.
 
 
 ### You use Linux v6.5 or below
 
-#### You need `mmap` (used by rtorrent and many sqlite3 base software)
+#### You need `mmap`
+
+To keep the details in a single location: see [Known Issues and
+Bugs#Software Using mmap](known_issues_bugs.md#software-using-mmap)
+for more details.
 
 * cache.files=auto-full
 * category.create=pfrd

mkdocs/docs/tips_notes.md

@@ -19,9 +19,10 @@
   compliant filesystems. mergerfs doesn't generally make exceptions
   for FAT, NTFS, or other non-POSIX filesystem.
 * Unless using Linux v6.6 or above do **not** use `cache.files=off` if
-  you expect applications (such as rtorrent) to use
+  you expect applications to use
   [mmap](http://linux.die.net/man/2/mmap). Enabling `dropcacheonclose`
-  is recommended when `cache.files=auto-full`.
+  is recommended when `cache.files=auto-full`. See
+  [quickstart](quickstart.md).
 * [Kodi](http://kodi.tv), [Plex](http://plex.tv),
   [Subsonic](http://subsonic.org), etc. can use directory
   [mtime](http://linux.die.net/man/2/stat) to more efficiently