Backport doc changes

docs/faq.rst

@@ -757,31 +757,25 @@ You will also get this error if you try to access a repository that uses the arg
 We recommend upgrading to the latest stable version and trying again. We are sorry. We should have thought abount forward
 compatibility and implemented a more helpful error message.
 
-Why does Borg extract hang after some time?
--------------------------------------------
-
-When I do a ``borg extract``, after a while all activity stops, no cpu usage,
-no downloads.
-
-This may happen when the SSH connection is stuck on server side. You can
-configure SSH on client side to prevent this by sending keep-alive requests,
-for example in ~/.ssh/config:
+Why am I seeing idle borg serve processes on the repo server?
+-------------------------------------------------------------
 
-::
+Please see the next question.
 
-    Host borg.example.com
-        # Client kills connection after 3*30 seconds without server response:
-        ServerAliveInterval 30
-        ServerAliveCountMax 3
+Why does Borg disconnect or hang when backing up to a remote server?
+--------------------------------------------------------------------
 
-You can also do the opposite and configure SSH on server side in
-/etc/ssh/sshd_config, to make the server send keep-alive requests to the client:
+Communication with the remote server (using an ssh: repo URL) happens via an SSH
+connection. This can lead to some issues that would not occur during a local backup:
 
-::
+- Since Borg does not send data all the time, the connection may get closed, leading
+  to errors like "connection closed by remote".
+- On the other hand, network issues may lead to a dysfunctional connection
+  that is only detected after some time by the server, leading to stale ``borg serve``
+  processes and locked repositories.
 
-    # Server kills connection after 3*30 seconds without client response:
-    ClientAliveInterval 30
-    ClientAliveCountMax 3
+To fix such problems, please apply these :ref:`SSH settings <ssh_configuration>` so that
+keep-alive requests are sent regularly.
 
 How can I deal with my very unstable SSH connection?
 ----------------------------------------------------
@@ -797,32 +791,6 @@ could try to work around:
   to do any more. Due to the way borg mount works, this might be less efficient
   than borg extract for bigger volumes of data.
 
-Why do I get "connection closed by remote" after a while?
----------------------------------------------------------
-
-When doing a backup to a remote server (using a ssh: repo URL), it sometimes
-stops after a while (some minutes, hours, ... - not immediately) with
-"connection closed by remote" error message. Why?
-
-That's a good question and we are trying to find a good answer in :issue:`636`.
-
-Why am I seeing idle borg serve processes on the repo server?
--------------------------------------------------------------
-
-Maybe the ssh connection between client and server broke down and that was not
-yet noticed on the server. Try these settings:
-
-::
-
-    # /etc/ssh/sshd_config on borg repo server - kill connection to client
-    # after ClientAliveCountMax * ClientAliveInterval seconds with no response
-    ClientAliveInterval 20
-    ClientAliveCountMax 3
-
-If you have multiple borg create ... ; borg create ... commands in a already
-serialized way in a single script, you need to give them ``--lock-wait N`` (with N
-being a bit more than the time the server needs to terminate broken down
-connections and release the lock).
 
 .. _disable_archive_chunks:
 

docs/quickstart.rst

@@ -311,36 +311,30 @@ Backup compression
 ------------------
 
 The default is lz4 (very fast, but low compression ratio), but other methods are
-supported for different situations.
+supported for different situations. Compression not only helps you save disk space,
+but will especially speed up remote backups since less data needs to be transferred.
 
-You can use zstd for a wide range from high speed (and relatively low
-compression) using N=1 to high compression (and lower speed) using N=22.
-
-zstd is a modern compression algorithm and might be preferable over zlib and
-lzma, except if you need compatibility to older borg versions (< 1.1.4) that
-did not yet offer zstd.::
+zstd is a modern compression algorithm which can be parametrized to anything between
+N=1 for highest speed (and relatively low compression) to N=22 for highest compression
+(and lower speed)::
 
     $ borg create --compression zstd,N /path/to/repo::arch ~
 
-Other options are:
-
-If you have a fast repo storage and you want minimum CPU usage, no compression::
+If you have a fast repo storage and you want minimum CPU usage you can disable
+compression::
 
     $ borg create --compression none /path/to/repo::arch ~
 
-If you have a less fast repo storage and you want a bit more compression (N=0..9,
-0 means no compression, 9 means high compression):
-
-::
+You can also use zlib and lzma instead of zstd, although zstd usually provides the
+the best compression for a given resource consumption. You may want to use these
+algorithms if you need compatibility to older borg versions (< 1.1.4) that
+did not yet offer zstd. Please see :ref:`borg_compression` for all options.
 
-    $ borg create --compression zlib,N /path/to/repo::arch ~
-
-If you have a very slow repo storage and you want high compression (N=0..9, 0 means
-low compression, 9 means high compression):
-
-::
+An interesting alternative is ``auto``, which first checks with lz4 whether a chunk is
+compressible (that check is very fast), and only if it is, compresses it with the
+specified algorithm::
 
-    $ borg create --compression lzma,N /path/to/repo::arch ~
+    $ borg create --compression auto,zstd,7 /path/to/repo::arch ~
 
 You'll need to experiment a bit to find the best compression for your use case.
 Keep an eye on CPU load and throughput.
@@ -408,7 +402,8 @@ is installed on the remote host, in which case the following syntax is used::
 
   $ borg init user@hostname:/path/to/repo
 
-Note: please see the usage chapter for a full documentation of repo URLs.
+Note: Please see the usage chapter for a full documentation of repo URLs. Also
+see :ref:`ssh_configuration` for recommended settings to avoid disconnects and hangs.
 
 Remote operations over SSH can be automated with SSH keys. You can restrict the
 use of the SSH keypair by prepending a forced command to the SSH public key in

docs/usage/serve.rst

@@ -44,6 +44,8 @@ locations like ``/etc/environment`` or in the forced command itself (example bel
 
 Details about sshd usage: `sshd(8) <https://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man8/sshd.8>`_
 
+.. _ssh_configuration:
+
 SSH Configuration
 ~~~~~~~~~~~~~~~~~
 ``borg serve``'s pipes (``stdin``/``stdout``/``stderr``) are connected to the ``sshd`` process on the server side. In the event that the SSH connection between ``borg serve`` and the client is disconnected or stuck abnormally (for example, due to a network outage), it can take a long time for ``sshd`` to notice the client is disconnected. In the meantime, ``sshd`` continues running, and as a result so does the ``borg serve`` process holding the lock on the repository. This can cause subsequent ``borg`` operations on the remote repository to fail with the error: ``Failed to create/acquire the lock``.

src/borg/archiver.py

@@ -2672,7 +2672,8 @@ def do_break_lock(self, args, repository):
             The heuristic tries with lz4 whether the data is compressible.
             For incompressible data, it will not use compression (uses "none").
             For compressible data, it uses the given C[,L] compression - with C[,L]
-            being any valid compression specifier.
+            being any valid compression specifier. This can be helpful for media files
+            which often cannot be compressed much more.
 
         obfuscate,SPEC,C[,L]
             Use compressed-size obfuscation to make fingerprinting attacks based on